The Collaborative International Dictionary of English v.0.48:

Touchback \Touch"back`\ (t[u^]ch"b[a^]k`), n. (Football)
   The act of touching the football down by a player behind his
   own goal line when it received its last impulse from an
   opponent; -- distinguished from safety touchdown.
   [1913 Webster]

WordNet (r) 3.0 (2006):

touchback
    n 1: (American football) a play in which the opposing team has
         kicked the football into your end zone