Luck

From Wiki Gonzalez

"Luck," when used in sabermetrics, does not refer to pink hearts, yellow moons, orange stars, green clovers, blue diamonds, or purple horseshoes, although they ARE magically delicious.

Rather, one must understand that everything that happens during a baseball game has a certain probability of occurring. When a rare occurrence happens, and is not obviously due to skill, then it can be explained by random variation in a baseball game. For example, a ball that hits the lip of the grass and bounces away from the shortstop is likely due to "luck" because this occurence is relatively rare, and is not attributable to the skill of the batter or the lack of skill of the shortstop.

Put another way - unusual outcomes will occur, that is a given. When these outcomes are not due to skill, they will occur at random, more or less. The random occurrence of rare outcomes is referred to as "luck."