|Definition - "The absence of an overt mark, written or spoken, as against its presence in corresponding positions elsewhere …." (OED).|
Example - The plural of dog is dogs. However, the plural of sheep is zero (i.e. sheep).
Oxford English Dictionary - Its first citation is from 1891:
"In Sanskrit Grammar, this ‘lopa’ is considered as a substitute or âde<sundl>a, and as such this grammatical zero has all the rights and liabilities of the thing it replaces."
(S. C. Vasu AshBádhyáyí of Pánini 56)