The Collaborative International Dictionary of English v.0.48:

Memento mori \Me*men"to mo"ri\ [L.]
   Lit., remember to die, i.e., that you must die; a warning to
   be prepared for death; an object, as a death's-head or a
   personal ornament, usually emblematic, used as a reminder of
   death.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

memento mori
    n 1: a reminder (as a death's head) of your mortality