The Collaborative International Dictionary of English v.0.48:

LTM \LTM\ n. (Psychology, Neurophysiology)
   long-term memory, the term for the set of neurons and neural
   processes responsible for the general store of information
   remembered for more than a day.

   Syn: long-term memory.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

LTM
    n 1: your general store of remembered information [syn: long-
         term memory, LTM]