Árbitro vs. réferi
Both árbitro and referí describe sports officials. The main difference is that árbitro is the standard, universal Spanish term used across all Spanish-speaking regions and can also apply to legal arbitration. In contrast, referí is an informal loanword from English used exclusively for sports in certain parts of Latin America.
Árbitro
The standard, formal, and universally accepted word for a referee or umpire in any sport, which can also be used to refer to an arbitrator, judge, or mediator in legal and professional disputes.
El árbitro pitó el final del partido de fútbol.
(The referee blew the whistle for the end of the soccer match.)
Contrataron a un árbitro imparcial para resolver el conflicto laboral de la empresa.
(They hired an impartial arbitrator to resolve the labor dispute of the company.)
Nunca es buena idea discutir con el árbitro durante un torneo de tenis.
(It is never a good idea to argue with the umpire during a tennis tournament.)
La federación decidió sancionar al árbitro por sus graves errores técnicos.
(The federation decided to sanction the referee for his serious technical mistakes.)
El juez actuó como árbitro entre las dos familias que peleaban por la herencia.
(The judge acted as a mediator between the two families fighting over the inheritance.)
Réferi
A colloquial adaptation of the English word referee, used primarily in some Latin American countries exclusively for sports, such as boxing, wrestling, or soccer, but never for legal or formal mediation.
El referí detuvo la pelea de boxeo en el tercer asalto.
(The referee stopped the boxing match in the third round.)
Los aficionados enojados le gritaron cosas horribles al referí después del penal.
(The angry fans yelled horrible things at the referee after the penalty.)
En la lucha libre mexicana el referí tiene un papel muy dramático.
(In Mexican professional wrestling the referee has a very dramatic role.)
Mi tío trabaja los fines de semana como referí en una liga de fútbol local.
(My uncle works on weekends as a referee in a local soccer league.)
El referí comenzó el conteo de diez cuando el peleador cayó a la lona.
(The referee started the ten count when the fighter fell to the canvas.)
Summary
In short, árbitro is the safest, most formal, and versatile word to use in any situation involving a judge, umpire, or referee. On the other hand, referí is a regional and casual anglicism that is fun and natural for sports conversations in Latin America, particularly in boxing and wrestling, but should be avoided in formal or legal contexts.