From BR Bullpen
A fly ball or fly is a ball hit with a high arching trajectory. It is distinguished from a ground ball, which bounces or rolls towards the outfield, and a line drive, which is hit with a straight trajectory. A fly ball that is caught before it hits the ground is an out, and most fly balls, except for those that are hit to the deepest part of the outfield or in the gap between two outfielders, are relatively easy to field, as players have time to position themselves where the ball will land. However, the vast majority of home runs are hit on fly balls, so there is some danger for a pitcher who has a tendancy to allow a lot of fly balls.
Fly balls are usually the result of a batter hitting the bottom part of a pitched ball, or using an uppercut swing (as a way of producing extra power, for example). Fastball pitchers will tend to give up a lot of fly balls, as a hard-thrown pitch will fall less than a batter expects, meaning that the batter will have a tendency to swing under it. This is the result of an optical illusion known as the "rising fastball".
A fly ball that is hit with an almost vertical trajectory is known as a pop-up. Pop-ups are very easy to field, except in very windy conditions or when there is confusion among a team's fielders. Because pop-ups are considered to be virtually automatic outs, baseball's most complex rule, the infield fly rule was devised to prevent teams on defense from benefitting unduly from pop-ups hit by their opponents.
When a batted ball is caught on the fly, runners who have left their bases must return to their bases or be called out.