∅

Etymology
The set-theoretic meaning of this symbol was introduced by the Bourbaki group in their treatise Éléments de mathématique. André Weil has said in his autobiography that he based it on the letter Ø.

Symbol

 * 1)  A set with no elements: the empty set.
 * 2)  A null morpheme; a theoretical morpheme used in models of linguistic analysis to represent the absence of a morpheme where one might otherwise be expected.
 * 1)  A null morpheme; a theoretical morpheme used in models of linguistic analysis to represent the absence of a morpheme where one might otherwise be expected.

Etymology
From the empty set symbol.