I don't know if there's an exact definition, but if I had to give some characteristics in order of importance:
1) Has defeated at least 1, if not several recognized premier fighters in their prime (my definition of premier is universally accepted HOF fighter)
2) Has defended the belt at least 1, if not several occasions, against recognized premier fighters (it's one thing when you have the hunger, but how many have the drive to stay on top and take the physical abuse after winning it all?)
3) Has won the majority of his fights (For me majority would be somewhere around 85-90%. If you winning every other fight your wins become meaningless)
4) Has at times dominated other boxers (it should be universally accepted that there was at least one, if not several fighters whose soul(s) you took)
5) Has defended his belt in different weight classes (not a requirement, but if you are defending your belt in different weight classes it goes to show you that it's you personally that is dominant, not some sort of physical advantage you may have on a smaller/slower boxer)
^^^It's not a complete set of criteria, but it's definitive. Everybody I consider GOAT has complied with my set of criteria, Mayweather hasn't unfortunately. Again, he's a great fighter, definitely an all-time great, definitely top 50, possibly top 20, but there is nothing he has done, and now nothing he can do to be considered the GOAT. He just hasn't had that kind of career, sorry.