A home run occurs when a batter hits a fair ball and scores on the play without being put out or without the benefit of an error.
In almost every instance of a home run, a batter hits the ball in the air over the outfield fence in fair territory. In that situation, the batter is awarded all four bases, and any runners on base score as well. The batter can circle the bases at his leisure, as there is no threat of him being thrown out. (This also occurs when the ball hits the foul pole in left or right field, or when the ball hits an opposing defender on the fly and bounces directly over the wall in fair ground.)
There are also instances of "inside-the-park home runs." These occur when the batter hits the ball in play (not over the wall) and touches all four bases without being thrown out. These are extremely rare and typically only occur with a very fast runner at the plate and some sort of misplay by an outfielder that doesn't qualify as an error.
Home runs can be a great stat for evaluating a hitter's power. They're also good for measuring the success of pitchers, who strive to limit home runs. But home run totals can be affected by the ballpark in which a game is being played. Some ballparks are smaller, have higher walls or have different wind currents. This means home runs in certain ballparks might have stayed in the yard in others.
The term "home run" comes from the basic act of a batter circling all the bases successfully. In the early days of the home run, running was typically a necessity as players weren't very powerful and outfields were much bigger, leading to a greater number of inside-the-park home runs. Now, however, most home runs feature players trotting around the bases after hitting the ball over the fence.
In A Call
"homer," "long ball," "dinger," "tater," "jack," "shot," "four-bagger," "blast," "big fly," "going yard," "going deep," "clout," "round tripper," "gopher ball" (used to describe the home run pitch thrown by the pitcher)