What is "null sign"

n. (context set theory English) The sign "∅", associated with empty sets.

In mathematics, the null sign denotes the empty set.

Common notations for the empty set include "{}", "∅", and "∅". The latter two symbols were introduced by the Bourbaki group (specifically André Weil) in 1939, inspired by the letter Ø in the Norwegian and Danish alphabets (and not related in any way to the Greek letter Φ).

In linguistics, the null sign is used to indicate the absence of an element, such as a sound or morpheme. For example, "I went to ∅ party" indicates that the usual determiner (perhaps "a" or "the") is missing.

It is also used in distinguishing the symbol null from zero (0).