A yard (abbreviation: yd) is the name of a unit of length in a number of different systems, including English units, Imperial units, and United States customary units. Its size can vary from system to system. The most commonly used yard today is the international yard which by definition is equal to 0.9144 metre.
The yard is used as the standard unit of field length measurement in both the American, English and Canadian games of football (although Canada has officially adopted the metric system).
Definition from Wikipedia:Yard see there for full article.