From BR Bullpen
A line drive is a batted ball hit in a relatively straight line. It is distinguished from a fly ball, which is hit with a high arching trajectory, and a ground ball, which is hit on the ground and either bounces or rolls towards the outfield.
A line drive is usually the result of a batter making good contact with a pitch. It is the hardest type of batted ball to field, as it is not slowed by contact with the ground and does not allow time for a fielder to position himself under the ball. As a result, pitchers who give up a lot of line drives do not last very long, while hitters who hit them consistently tend to have high batting averages. While a line drive will occasionally clear the outfield fence and become a home run, it is more likely for it to land on the ground somewhere in the outfield, or to hit the fence. Rare as they are, line drive home runs are usually quite spectacular, as the ball needs to be hit extremely hard and will leave the playing field as if shot from a cannon.
A line drive which is caught is an out. It also requires runners who have left their bases to return to them or be called out. A caught line drive will often result in a double play as the runners have little time to react and retreat to safety as the ball is usually caught a fraction of a second after being hit. Most triple plays are also the result of line drives.
A line drive that is hit very hard is often referred to as a "scorching" or "screaming" line drive. These can be dangerous if they are hit back towards the pitcher, who is often out of position after finishing his delivery and unable to protect himself, or at third base, which is known as "the hot corner" because of the frequency of such drives which can only be fielded by a third baseman with excellent reflexes. A "soft line drive" is usually one that is hit at the end of the bat, towards the opposite field (i.e towards first base for a right-handed batter or towards third base for a left-hander). A soft line drive that lands just behind the infield for a base hit is known as a bloop hit.