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.
There has been so much negativity with the term “master” trainer that I now explain it as being capable of providing instruction that will directly lead to certification or recertification as a fitness instructor. It can be a general or specific teaching area in our profession. A master trainer could be a indoor cycle class master trainer, but not a kick boxing master trainer, etc.
Pretty much, if you have the skill and ability to teach others how to lead a fitness program and have demonstrated that ability by actually teaching others to safely and successfully instruct clients in a fitness program, you should be considered a master trainer.