The Collaborative International Dictionary of English v.0.48:

cytosine \cy"to*sine\ (s[imac]"t[-o]*s[i^]n or
   s[imac]"t[-o]*s[=e]n), n. (Biochemistry)
   A pyrimidine (C4H5N3O) which is one of the four major basic
   components of DNA and RNA in most organisms, forming
   glyosides with ribose and deoxyribose. It is the basic
   component of cytidine, deoxycytidine, cytosine, cytidine
   monophosphate, and derivatives of those compounds.
   [PJC]

WordNet (r) 3.0 (2006):

cytosine
    n 1: a base found in DNA and RNA and derived from pyrimidine;
         pairs with guanine [syn: cytosine, C]