FitNesse. UserGuide.
MarkupVariables [add child]

Defining and Expressing Variables.

You can define a variable to represent any amount of text. The text in a variable is never interpreted as wiki markup. It is always raw literal text.

!define myVariable {
This is the
text of my

variable defined: myVariable= this is the text of my variable
You express the value of a variable like this: ${myVariable}. This expression is replaced with the value of the variable.

The value of the variable is:
this is the
text of my

If a variable is expressed on a page, but is not found, then its parent pages are searched in order.
If the variable is not found in any ancestor pages, FitNesse will inspect

There are several pre-defined variables[?] that you can use on your pages.

See Variable fitnesse test, Parent Variable fitnesse test and Define Variables in System Properties test