The Collaborative International Dictionary of English v.0.48:

information theory \in`for*ma"tion the"o*ry\, n. (Math.,
   Telecommunications)
   The science which studies the capacity of systems to contain
   and transmit information[2], and the factors such as noise
   and channel capacity that may affect the rate or accuracy of
   information transmission and reception.
   [PJC]

WordNet (r) 3.0 (2006):

information theory
    n 1: (computer science) a statistical theory dealing with the
         limits and efficiency of information processing