In baseball, a catch occurs when a fielder gains secure possession of a batted ball in flight, and maintains possession until he voluntarily or negligently releases the ball. When a catch occurs, the batter is out, and runners, once they properly tag up (retouch their time-of-pitch base), may attempt to advance at risk of being tagged out.
Unlike in American football and other sports, neither secure possession for a time nor for a number of steps is enough to demonstrate that a catch has occurred. A fielder may, for example, appear to catch and hold a batted ball securely, take a few more steps, collide with a wall or another player, and drop the ball—this is not a catch.
To avoid ambiguity with the common term catch meaning any action that gains possession of a ball, some may say that a fielder gloved a thrown ball or a batted, bouncing ball.
Cricket
A catch in cricket is similar to a catch in baseball. The fielder must obtain control over the ball and his own movement, without crossing the boundary. See the article Caught for more details.
In fisheries science, by-catch refers to species caught in a fishery intended to target another species, as well as reproductively-immature juveniles of the target species.
By-catch is a serious issue that can contribute to species endangerment.
This by-catch issue has been one of the reasons of the growing eco-labelling industry, where fish producers mark their packagings with something like "Dolphin Friendly" to reassure buyers.