资料来源 : Webster's Revised Unabridged Dictionary (1913)

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.

资料来源 : WordNet®

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