Wordnet 3.0
NOUN (1)
1. 
 the 13th letter of the Hebrew alphabet; 
The Collaborative International Dictionary of English v.0.48:
mem \mem\ n.
   the 13th letter of the Hebrew alphabet.
   [WordNet 1.5]
WordNet (r) 3.0 (2006):
mem
    n 1: the 13th letter of the Hebrew alphabet