DEFINED
UNSET

Syntax: DEFINED (anything)
   and: UNSET (anything)
Location: BTool (DEFINED)
          PARAMS (DIY Toolkit) (UNSET)

The function DEFINED takes any parameter, no matter what type it is, provided that it is a constant or a variable.

DEFINED returns 0 if the parameter was a variable but unset and 1 for defined variables and constant expressions.

UNSET is the same as DEFINED.

CROSS-REFERENCE:
CLEAR makes all variables undefined.  
PRINT writes asterisks if unset variables are required to be printed.  
TYPE returns 1, 2 or 3 for undefined variables.
