The Collaborative International Dictionary of English v.0.48:

Mores \Mo"res\ (m[=o]"r[=e]z), n. pl.; sing. Mos (m[=o]s).
   [L.]
   Customs; habits; esp., moral customs conformity to which is
   more or less obligatory; customary law. [singular is rarely
   used]
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

mores
    n 1: (sociology) the conventions that embody the fundamental
         values of a group