[techspeak] A name (possibly followed by a formal arg list) that is equated to a text or symbolic expression to which it is to be expanded (possibly with the substitution of actual arguments) by a macro expander. This definition can be found in any technical dictionary; what those won't tell you is how the hackish connotations of the term have changed over time.
The term macro originated in early assemblers, which encouraged the use of macros as a structuring and information-hiding device. During the early 1970s, macro assemblers became ubiquitous, and sometimes quite as powerful and expensive as HLLs, only to fall from favor as improving compiler technology marginalized assembler programming (see languages of choice). Nowadays the term is most often used in connection with the C preprocessor, LISP, or one of several special-purpose languages built around a macro-expansion facility (such as TeX or Unix's [nt]roff suite).
Indeed, the meaning has drifted enough that the collective macros is now sometimes used for code in any special-purpose application control language (whether or not the language is actually translated by text expansion), and for macro-like entities such as the keyboard macros supported in some text editors (and PC TSR or Macintosh INIT/CDEV keyboard enhancers).
