undef

Adjective

undefined (not comparable)

1. Lacking a definition or value.
   • We slid into our office, past two women discussing the "X factor," an apparently undefined quality their firm felt they lacked.1992, Lia Matera, Prior Convictions, page 121

   Synonyms:

   indefinite

2. That does not have a logically sound meaning within a specific formal system and is thus not assigned an interpretation.
   • The result of division by zero is undefined.
3. That behaves unreliably due to dependence on an uninitialized value or inconsistently ported code.
   • In C, relying on a variable's value right after a malloc call is undefined behvaior.

See all 4 meanings of undefined