The Collaborative International Dictionary of English v.0.48:

libido \li*bi"do\ (l[i^]*b[=e]"d[-o]), n.
   1. Sexual desire; the sexual instinct; the sex drive.
      [PJC]

   2. (Psychoanalysis) Those desires and instinctual energies
      which are derived from the id.
      [PJC] Libken

WordNet (r) 3.0 (2006):

libido
    n 1: (psychoanalysis) a Freudian term for sexual urge or desire