Accounting Framework - implements conservation rules
Binding Variable - A simulator variable which is linked to two different variables from the calibrator: one which is used when the model is run over simulation time, and a different one for historical time. Binding variables are often base-year variables. Also note that binding variables are fixed - their values cannot be set from within a simulator framework - but not all fixed variables are binding. See the definition of fixed variables.
Calculator - Is the lowest level node in the conceptual hierarchy; it consists of one or more procedures that effect the transformation of input objects into output objects; it is the smallest unit that is individually executable. It can be considered as a sub-model within the larger model.
Callibration- The specification and estimation of a macroeconomic model differs from the design and calibration of a design approach model. In particular, the use of data is different as in macroeconomic modelling the data are used to estimate the parameters in the model using an appropriate statistical technique, while in the design approach, data which have not been measured, such as lifetimes of capital stocks, are estimated and then the data set is used to calibrate the model. The calibration is successful if the model reproduces the data, given a set of historical input variables, and this is considered a necessary but not sufficient condition for a valid model. [make this defn a bit more stand-alone and clearer]
Conceptual Hierarchy - is a hierarchical structure consisting of nodes that is used to organize information pertaining to a framework. The highest level node is the framework itself; a framework may have one or more sub-framework nodes, and sub-framework nodes may have one or more sub-framework nodes and/or one or more calculators
Concordance - Bert
Control Space- In the design approach, processes are separated into two classes: those that transform information and those that transform energy and material. Models of processes dealing only with information-and these include decision processes -occupy the control space of systems theory, while models of processes dealing with energy and material occupy machine space, but with their control function linked to control space). 
Control Variable is a variable that is an input to a calculator that can be changed by the user of the framework in order to create a new scenario. A control variable may be a parameter, a stock, or a flow.
Database model - a model (or framework) which runs over an observed historical period, usually with the purpose of assembling an internally-consistent historical database. Its conceptual purpose can be the same of a calibrator framework except that a database model is not linked to a counterpart simulator framework.
Dependency Structure - represents the flows of information among the calculators of a framework and takes the form of an acyclic graph. The computational sequence of calculators may be inferred from the dependency structure.
Design Approach - is an approach to modelling distinguished by the interaction between an open simulation framework and a user of the framework who enhances his understanding of the system by exploratory simulation and who is the source of novelty
Design Diagram - is a diagrammatic representation of the variables, procedures, and the relationships among them that constitute a calculator
Endogenous(variable) - Data is calculated within the model. See Also: Output Variable
Equivalence Rules - are the mappings of the output variables of a calculator onto the input variables of other calculators as well as the equivalence of input variables into two or more calculators. The dependency structure is deduced from the equivalence rules.
Exogenous (variable) - Data is not calculated within the model, instead is provided by the user/controller. See Also: Control Variable
Extensive (variable) - property of a system that does depend on the system size or the amount of material in the system. Wikipedia page for intensive and extensive properties.
Feedback - Multiple definitions: systems theory feedback (thermostat), feedback view (equilibrating or computational) and feedback view (user expectation). See Also: Weak Feedback, Strong Feedback
Feedforward - defn here. Hard-wired model connection. See also: Coordination.
Fixed Variable is a variable that is an input to a calculator that cannot be changed by the user of the framework, usually a universal constant such as the speed of light or the starting value of a stock.
Flow - is a rate variable that is expressed in units per unit time; flow variables are represented in design diagrams as pipes
Framework is a set of calculators organized by a conceptual hierarchy and linked together through a dependency structure, which when taken together represent the processes that constitute a system
Framework Variable - A variable used in more than one calculator (i.e. across the framework). In model diagrams these variables are identifiable by their yellow connectivity tags.
Header is the part of an object that contains information about the shape of the object
Historical Binding - A property of a dynamic system such that some or all of the system's initial conditions are solely determined (or bound) by its historical evolution. Also see Binding Variable. - Bert
Informant - is an object or a virtual object that gives form to another object or variable. Objects may be informed by one or more informants. There are many possible types of informants. Three commonly used types are set, sequence and category
Instance - is a specific set of values or a specific filling of a variable; a variable may have more than one instance; an instantiated variable is an object.
Intensive (variable) - is a physical property of a system that does not depend on the system size or the amount of material in the system: it is scale invariant. Wikipedia page for intensive and extensive properties.
Load Factor (LF) for electricity production capacity - The load factor is the ratio of the amount of electricity produced by a technology class in a year to the amount of electricity that could be potentially produced if the capacity of the technology class were run for every day of the year 24 hours per day. It is unitless and is a measure of how much electricity demand was dispatched (or assigned) to that technology class in a year. Dispatch in an actual grid of electricity production capacity is done on a minute by minute basis.
Machine Space- (also see Control Space).
Markovian - A markovian process is one whose next period state depends only on its state in the last time period
Model is a representation of a system
Node - is a point in the conceptual hierarchy that may refer to a framework, a sub-framework, or a calculator.
Over-determined - System property - Rob
Output (variable) - a variable whose contents is calculated in a procedure, as opposed to an input variable whose content is set by the user (potentially via a view).
Parameter - is a ratio variable; parameters are represented in design diagrams as flattened hexagons
Procedure - is an equation, set of equations, or an algorithm that effects the transformation of values of input variables into values of output variables; procedures are represented as rectangles in design diagrams
Process - is a transformation of some mixture of information, energy and material into another such mixture.
Scenario - is a set of instances for all the variables of a framework that are consistent with the relationships of the framework.
Sequence - is an informant that is a list of elements with an imposed “next to” relationship between its elements. As an object, a sequence is virtual because the list of elements need not be explicit; only the beginning, the end and the “next to” rule need be represented
Set - is an informant that is a list of elements in which the ordering of the elements is arbitrary.
Shape - is a definition of the attributes of an object or variable in terms object type (array, scalar, set), data type (real numbers, integers), the number of dimensions and the dimension name(s), its standard international unit of measure, its alternate unit of measure, and its entity. Information pertaining to shape resides in the header of each object.
Simulation Model (also: Simulation Framework, Simulator) - is a framework used to create future scenarios of the system represented by the framework. A simulation framework is an open model or incomplete representation of the system in question. The representation of the system is completed through interaction between the framework and the user of the framework.
State Space Model -
Stock - is a variable that is a measure of what exists or a level at an instant in time; stock variables are represented in design diagrams as barrels (vertical cylinders).
Strong Feedback - is interdependence among processes that are represented within the same calculator. The transformation of input values to output values in a calculator with strong feedback requires the solution of a set of simultaneous equations
Systems Model - is a formal description of the understanding needed by a manager or controller of a system for effective control of a controlled system. It is a representation of the controlled system in the context of its environment.
Tension - Simulations can produce physically inconsistent or socially unacceptable futures. For example, a scenario to study technological change might be deemed inconsistent because of a projected shortage of systems design engineers . The notion of explicit model tension contrasts that of enforced equilibrium models. Tension is a measure of the discordance among the processes represented by the calculators or components of a framework for a particular scenario. Tension may be resolved by the framework user through the specification of weak feedback loops.
Tool - Tools are specific functions within the TOOL language.
TOOL - the multi-dimensional array language used throughout the whatIf Platform. The TOOL language is interpreted, and contains individual compiled tools (or functions). TOOL is developed and maintained by whatIf? Technologies.
Variable - is a container for information pertaining to the properties of a system; variables are used to express the relationships among the properties of a system. Each variable has a name that is defined within the context of a particular calculator or framework. Variables are designated according to three criteria: (i) relational type: fixed, control or output; (ii) framework type: specific to calculator or specific to framework, and; (iii) class: stock, flow or parameter.
View - a procedure that is designed to facilitate the display or creation of the contents of a variable or group of variables. Technically, a view is implemented as a TOOL script loaded in a model family.
Weak Feedback - is interdependence among processes that involves interdependence among calculators of a framework. Weak feedback loops may be resolved by the framework user through repeated simulation or by the creation of a second framework linked to the first framework. The second framework takes as input selected outputs from the first framework and produces as output values of selected input variables to the first framework. Solution of the two frameworks can be reached through iteration.
 Gault, F. D., K. E. Hamilton, R. B. Hoffman, and B. C. McInnis. The Design Approach to Socio-Economic Modelling. Futures, February, 1987. http://www.whatiftechnologies.com/publication/THE_DESIGN_APPROACH_TO_SOCIO_ECONOMIC_MODELLING.pdf