The double team is a defensive tactic in many sports, for example American football and basketball. A player on offense is being double-teamed when s/he is covered by two defensive players.

Usually star players are double-teamed in basketball, however this leaves one offensive player uncovered, or open. If the covered player has the ball at the time, then the hope is to force a turnover or error. On the other hand, in college, high school, and international leagues, one can double team an offensive player without the ball, providing extra coverage. For a long time this was not allowed in the NBA since teams had to play a man to man defense, with the only exception being the double teaming of the ball handler. (See this node for an interesting story.) As hashbrownie reminded me, however, zone defense will be allowed back in the NBA for the 2002 season.

In football wide receivers are often double-teamed by a cornerback and a safety. The defense can usually afford to use two players to cover one, since they may not have as many people rushing as are on the offensive line. A double-teamed wide receiver is often said to be in double coverage.