A unanimous decision is a winning criterion in several full-contact combat sports, such as boxing, kickboxing, Muay Thai, mixed martial arts and others sports involving striking in which all three judges agree on which fighter won the match.
In boxing, each of the three judges keep score (round by round) of which fighter he / she feels is winning (and losing). A decision isn't required to be unanimous for a boxer to be given a victory. Unaninmous decision shouldn't be confused with a majority decision or split decision.
A unanimous decision is also sometimes referred to as a "win on points".