What are umpires?:
The on-field officials in baseball are called umpires, and they're the men or women with ultimate authority of the game, from the simple (out or safe, ball or strike) to the complex (rules interpretation). Umpires are paid at almost all levels of baseball.
Professional vs. amateur umpires:
In a Major League Baseball game, there are at least four umpires, one at each base (home plate, first, second and third). There could be as many as six in the playoffs (one down each fair-foul line in the outfield).
In the minor leagues there are fewer umpires (typically there are three, with no umpire at second base), and there are even fewer in the amateur ranks, all the way down to one for many amateur games.
When there are multiple umpires, they divide their roles.
- Plate umpire: In most instances, the plate umpire is the head umpire. He or she calls balls and strikes behind the catcher and fair/foul balls short of the bases. This umpire also keeps the supply of baseballs and decides if the field is playable if the weather is poor.
- Base umpires: The first bases and third base umpires make fair/foul calls past the base and make out and safe calls at the bases. They also decide appeals on swinging strike calls. If there are two umpires, one base umpire will roam, depending upon the situation.
"Umpire" is often shortened to ump, and another generic term for an umpire is "blue," which refers to the light blue shirts that are part of many basic umpire uniforms.
There's no room for debate with an umpire while a game is ongoing, even though you are likely to see managers and players arguing with an umpire from time to time.
But an umpire has complete authority over the game. And they have the power to eject anybody who doesn't agree, whether it's a player, coach or an unruly spectator.