NOUN (1)

1. a swimming stroke that resembles the crawl except the swimmer lies on his or her back;


VERB (1)

1. swim on one's back;

The Collaborative International Dictionary of English v.0.48:

backstroke \back"stroke`\ n.
   1. a swimming stroke that resembles the crawl except the
      swimmer lies on his or her back. It is usually executed
      with backward-moving circular arm strokes and a flutter
      kick.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

backstroke
    n 1: a swimming stroke that resembles the crawl except the
         swimmer lies on his or her back
    v 1: swim on one's back