EDIT Mathematics: Empty Set or Nul Set.

In the set theory of mathematics, this character represents an empty set or nul set, i. e. a set void of elements. It was introduced in set theory in 1939 by mathematicians Nicolas Bourbaki and André Weil.

Typographically, this character represents a circle with stroke and is not to be confounded with the Scandinavian letter Ø U+00D8 LATIN CAPITAL LETTER O WITH STROKE or with ⌀ U+2300 DIAMETER SIGN.

 85.20.221.234  :  de