The Collaborative International Dictionary of English v.0.48:

Latchkey \Latch"key`\, n.
   A key used to raise, or throw back, the latch of a door, esp.
   a night latch.
   [1913 Webster]

WordNet (r) 3.0 (2006):

latchkey
    n 1: key for raising or drawing back a latch or opening an
         outside door