A variable is a name that can hold a value. Although any variable can be set by the user, built-in variables typically exist specifically so that users can change them to alter the way Octave behaves (built-in variables are also sometimes called user options). Ordinary variables and built-in variables are described using a format like that for functions except that there are no arguments.
Here is a description of the imaginary variable
If the value of this variable is nonzero, Octave will do what you actually wanted, even if you have typed a completely different and meaningless list of commands.
Other variable descriptions have the same format, but `Built-in Variable' is replaced by `Variable', for ordinary variables, or `Constant' for symbolic constants whose values cannot be changed.