def and rdef
define a macro
SYNOPSIS
def macname string_constant
rdef macname expression
DESCRIPTION
The command def defines a macro named macname to be string_constant. Each time macname occurs on input, it will be replaced by string_constant. The definition is made at the time the command is first read, so the macro can be used later in the same statement block in which it is defined and can be redefined within the same statement block.
The command rdef is similar to def, but the macro assignment is not made until the encompassing statements are parsed and executed. Also, although expression is usually a string constant, it needn't be.