Base runner

In baseball, base running is the act of running around the bases performed by members of the team at bat.

In general, base running is a tactical part of the game with the goal of eventually reaching home to score a run. The goal of batting is generally to produce base runners, or help move base runners along. Runners on second or third base are considered to be in scoring position since a normal hit, even a single, will often score them. Part of the goal of a runner and a batter is to get the runner into scoring position.

For any base running to occur, a batter must initially become a base runner. This happens when:

The term batter-runner is used in official terminology to identify an offensive player from the time he puts a fair ball into play or the third strike is not caught (thereby ceasing to be a batter) until the end of the play he initiated, whether the play results in the player being put out or becoming a runner by legally attaining first base or any subsequent base. The term is generally not applied if the batter hits a foul ball or to a player awarded first base, e.g., for a base on balls.

A player ceases to be a base runner when:

If a base runner's teammate is put out for the third out of the inning, he is said to be left on base (LOB).

A runner who is touching a base which he is entitled to occupy may not be tagged out. Runners may attempt to advance from base to base on any fair ball that touches the ground. When a ball is hit in the air (i.e., a fly ball) and caught by the defending team, runners must return and touch the base they occupy—called tagging up—after the ball is first touched by a fielder. Once they do this, they may attempt to advance at their own risk. On a ball that touches the ground in fair territory, if there is a force, runners are required to run.

Base runners may attempt to advance at any time while the ball is alive, even before or while the pitcher is throwing a pitch. The catcher—or pitcher, in lieu of delivering the pitch—often tries to prevent this by throwing the ball to one of the infielders in order to tag the runner. This pick-off attempt is usually unsuccessful in tagging out the runner but is effective in keeping the runner closer to the base. If the runner is tagged out while diving back to the base, it is called a pickoff. If the runner attempts to advance to the next base but is tagged out before reaching it safely, he is caught stealing. A successful attempt by the runner is called a stolen base. If a pitch gets away from the catcher, runners may also try to advance. This may be a wild pitch, if the pitcher is held responsible for the ball getting away, or a passed ball if the catcher is deemed to be at fault. Sometimes the defending team will ignore a runner who is trying to steal a base; in this case a runner is not credited with a steal, and the base is attributed to defensive indifference.

...
Wikipedia