The Collaborative International Dictionary of English v.0.48:

directionality \directionality\ n.
   the property of a microphone or antenna of being more
   sensitive to signal arriving from one direction than from
   another.

   Syn: directivity.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

directionality
    n 1: the property of a microphone or antenna of being more
         sensitive in one direction than in another; "the antenna
         received signals with no directivity" [syn: directivity,
         directionality]
    2: the property of being directional or maintaining a direction;
       "the directionality of written English is from left to right"