rectrix

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One of the stiff main feathers of a bird's tail, used to control the direction of flight.

Century Dictionary and Cyclopedia

1. n. Same as rectoress.
2. n. In ornithology, a tail-feather; one of the long or large quill-feathers of a bird's tail: so called from its use in directing or steering the course of a bird in flight, like a rudder. The rectrices are comparable to the similar large flight-feathers of the wing, called remiges. In the Saururæ, or Jurassic birds with long lizard-like bony tail, the rectrices are biserially or distichously arranged in a row on each side of the caudal vertebræ. In all modern birds they are set together in a fan-like manner upon the pygostyle. (See Eurhipidura.) In a few birds they are rudimentary, as in grebes. The most frequent number by far is twelve, which prevails (with few anomalous exceptions) throughout the great order Passeres, and also in very many other birds of different orders. In many picarian birds the number is ten; in a very few eight. In various water-birds the rectrices run up to higher numbers, twenty-four being probably the maximum. There is normally always an even number, these feathers being paired. In size, shape, and texture they are endlessly varied, giving rise to all the different shapes a bird's tail presents.

Wiktionary

1. n. A governess; a rectoress.
2. n. A flight feather on the tails of birds, used for directional control.

GNU Webster's 1913

1. n. A governess; a rectoress.
2. n. (Zoöl.) One of the quill feathers of the tail of a bird.