swallowtail

1. 1. The deeply forked tail of a swallow.
   2. Something similar to the tail of a swallow.
2. See tailcoat.
3. Any of various colorful, widely distributed butterflies of the family Papilionidae, usually having an extension at the end of each hind wing that resembles the tails of certain swallows.

1. Dovetail moulding or triangular fret moulding consisting of a series of adjacent equilateral triangles, each set alternately with apex and base uppermost, the series framed by a system of fillets joining at acute angles, and decorating a band.

2. Guelphic crenellation. See battlement.

Swallowtail. Romanesque swallow-or dovetail moulding, also called triangular fret, from Ely Cathedral, Cambs. (After Parker)

The noun has one meaning:

Meaning #1: a man's full-dress jacket with two long tapering tails at the back
  Synonyms: swallow-tailed coat, morning coat