from all I know there is no one definition for it. There are some organizations like NASM which have a Master Trainer program. In their case, the candidates have to satisfy certain prerequisites and then demonstrate their knowledge to a panel. Other programs may have similar paths. A master trainer is often referred to as a person who is authorized to train others to be trainers.
Actually this is a great question. Just because someone has this tile behind their name doesn’t make them a master trainer. Without taking away anything from those who are true masters of this profession, I will add that during my career I have met trainers who were referring themselves as “Master Trainers” but lucked the experience to prove it.
Often the term “master trainer” is used in the context of trainers who provide education to other trainers, often associated with a group exercise format or a piece of equipment. Often, these individuals provide certifications or trainings in that format / equipment.
Companies have very different requirements regarding education, experience, and expertise.