The Definition attribute contains an Analytica expression that gets evaluated to obtain the Result of the variable. Definition attributes appear for objects types that are types of variables -- decision variables, change variables, general variables, objective variables, indexes, constants, or constraints. Definition also is used to define a User-Defined Function.
The simplest Definition is a literal value, such as a single number or a text value surrounded by quotes. When a variable has a simple literal Definition and has an input node, the input node takes the form of a text box.
A Definition may contain certain special forms which display as buttons or controls in the Object Window and Attribute panel when you view the Definition. These include Lists, Lists of Labels, Tables, Distributions, and Sequences. In all these cases, you can also view the Definition textually by selecting expr in the Definition-type pulldown.
Definitions may also contain arbitrary expressions of arbitrary length. To make your models easy to understand, it is best to keep these relatively short.