noun / verb
umpire
UM-pyre/ˈʌmpaɪər/
An independent official deciding according to a sport's rules.
The masked umpire signals a strike behind the catcher.

Etymology
“Not equal” meant an odd number: a third person brought in to arbitrate between two others. This dispute-settling role extended to the official who enforces a game's rules.
In context
once he stepped into into out of the runner's lane at 45 ft. At that point, as an umpire, I can always just say, "Nope, he was he was out of the baseline."
More examples
- The tennis player asks the umpire to explain the ruling.
- She umpires youth baseball games on weekends.
- Two umpires watch the play from different angles.
Forms
umpire · umpires · umpired · umpiring