Here are two senses for zero:
Robins, R. H. 1971.General linguistics: An introductory survey. 2nd edition. London: Longman.
Fleming, Ilah. 1988.Communication analysis: A stratificational approach. A field guide for communication situation, semantic, and morphemic analysis. Volume 2. Dallas, TX: Summer Institute of Linguistics.
Crystal, David. 1980.A first dictionary of linguistics and phonetics. Boulder, CO: Westview.