antenna

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Zoology One of the paired, flexible, segmented sensory appendages on the head of an insect, myriapod, or crustacean functioning primarily as an organ of touch.
• noun Something likened to this sensory appendage, as in function or form.
• noun A metallic apparatus for sending or receiving electromagnetic waves.

from The Century Dictionary.

• noun One of the lateral articulated appendages occurring in pairs on that segment of the head of an arthropod animal, as an insect, which immediately precedes the mouth or mandibular segment; a feeler or ‘horn.’
• noun An analogous organ on the heads of other animals, as a feeler or tentacle, like the eye-stalk of a snail.
• noun plural Projecting horns of iron or bronze found on some ancient helmets, perhaps serving only as ornaments, or as badges, or in some cases to stop a blow from glancing downward and striking the shoulder.
• noun In Rotifera, a spur-like process bearing a tuft of setæ and projecting from the mid-dorsal line close to the trochal disk. Same as calcar, 4.
• noun In electricity, the vertical conductor used in wireless telegraphy to send out electric waves (sender) or receive them (receiver).

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) 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.
• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A feeler organ on the head of an insect, crab, or other animal.
• noun An apparatus to receive or transmit radio waves and convert respectively to or from an electrical signal.
• noun The faculty of intuitive astuteness.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an electrical device that sends or receives radio or television signals
• noun sensitivity similar to that of a receptor organ
• noun one of a pair of mobile appendages on the head of e.g. insects and crustaceans; typically sensitive to touch and taste