A boolean variable is one that can only represent a true or false value.

The variable type was named after George Boole.

Note that, in Interchange configuration directives parlance, boolean is used somewhat awkwardly. Instead of adhering to above definition of boolean, it actually signifies an array in which you can search for the presence or absence of a value. Real boolean variables are called yesnos in Interchange.

This naming confusion is unfortunate, but is fortunately quarantined to the configuration directives "space".