The Collaborative International Dictionary of English v.0.48:

Antenna \An*ten"na\, n.; pl. Antenn[ae]. [L. antenna
   sail-yard; NL., a feeler, horn of an insect.]
   1. (Zool.) A movable, articulated organ of sensation,
      attached to the heads of insects and Crustacea. There are
      two in the former, and usually four in the latter. They
      are used as organs of touch, and in some species of
      Crustacea the cavity of the ear is situated near the basal
      joint. In insects, they are popularly called horns, and
      also feelers. The term in also applied to similar organs
      on the heads of other arthropods and of annelids.
      [1913 Webster]

   2. (Electronics) A metallic device, variously shaped,
      designed for the purpose of either transmitting or
      receiving radio waves, as for radio or television
      broadcasting, or for transmitting communication signals.
      Some types are: whip antenna, antenna tower, horn
      antenna, dish antenna, directional antenna and
      rabbit ears. See transmitter, receiver.
      [PJC]

WordNet (r) 3.0 (2006):

directional antenna
    n 1: an antenna that transmits or receives signals only in a
         narrow angle