An Etymological Dictionary of Astronomy and Astrophysics

1. The cardinal number of a class that has no members.
   

2. The only integer that is neither negative nor positive.
   

3. The → identity element for addition.

See also: From Fr. zéro, from It. zero, from M.L. zephirum, from Ar. sifr (صفر) “cipher,” translation of Skt. śūnya “empty place, void, naught.” The rules governing the use of zero appeared for the first time in the Indian mathematician Brahmagupta’s book Brahmasputha Siddhanta “The Opening of the Universe,” written in 628.