In a baseball game, the "starting pitcher," often shortened to "starter," is the pitcher who throws the first pitch of the game. Any pitcher who enters the game thereafter is referred to as a "relief pitcher" (or "reliever" for short). The term "starting pitcher" is also used to refer to a pitcher on a baseball team who usually starts games rather than entering as a reliever.
Compared to relief pitchers, starting pitchers are generally expected to pitch for more innings. This is reflected in the statistics themselves, as in order to be credited with a "win" (a stat commonly used to evaluate pitchers), a starter must pitch at least five full innings, where as a reliever can pitch only one inning, or sometimes even less, and still be credited with a win.
However, in exchange for pitching longer in each outing, starting pitchers are asked to pitch less often. Most major league baseball teams have 5 starting pitchers on the team at any one time, pitching them in a set sequence known as a "starting rotation," in order from the best pitcher, known as the "ace" or "number one starter," down to the fifth best, or "number five starter." This means that at most, a starting pitcher will be asked to pitch every five days, whereas many relievers are asked to pitch every other day or sometimes even on back-to-back days.
In addition to the "win," another baseball statistic that specifically refers to the starting pitcher is the "quality start," which is defined as a starting pitcher pitching at least six innings and allowing three runs or less, reflecting the widespread assumption that a decent starting pitcher should usually be able to pitch for at least six innings per start.
In addition to "quality starts," other stats which can only be accumulated by starting pitchers include the "complete game," in which a pitcher throws every pitch for his team for an entire game, the "shutout," in which a pitcher throws a complete game and allows no runs, the "no-hitter," in which a pitcher throws a complete game and allows no hits, and the "perfect game," in which a pitcher throws a complete game and allows no opposing batters to reach base by any means.
Given their high profile and large impact on most games in which they pitch, starting pitchers are usually the most prominent pitchers on their team, and most of the most famous pitchers in baseball history have been starting pitchers.