A variable is a construct that represents a non-constant value. Variables are used in parameter values. This enables changing one or more parameter values by changing a single variable value. For example, you could define an expression variable named "radius", assign it a value, and use it in a cylinder's radius parameter. Then you could define the cylinder's height parameter as "2 * radius". Changing the radius variable causes the radius and height parameters of the cylinder to update. This is called parameterizing geometry.
In addition to expression variables, Analyst supports variables over sets of predefined user choices, and over files. For further discussion, see here.
A variable can be used in any number of parameters.