I think the definition of ‘Master’ can vary between an individual. As an example, I consider Guthrie Govan a master of his craft because he can play alot of genres convincingly well. You send him to play a country gig and people would think he dedicates his time and practice to study country. The problem is that some musicians tend to approach everything and to tell the truth, dont really master their craft. Alot of musicians are considered masters in one genre. Paco De Lucia for example was a master of Flamenco and classical, but possibly couldnt play funk or heavy metal. I believe the saying ‘Jack of all trades, master of none’ is fitting.
In terms of Academia, you have musicians who have studied masters degrees who earn the title of master if they so wish. This is then followed by the qualification and title of PhD and Doctor. So in this, the term Doctor is more complimenting.
But also, consider that expert and master probably means the same thing. If you master a craft or genre that is your expertise.
If you’re asking how to progress to such a level, as well as practicing efficiently, read a lot of books and music. Study your theory and work hard. You don’t have to necessarily know every thing to become a master, but to be confident and masterful in one genre is sometimes the way forward (depends on your goals I suppose). We all can’t be Guthrie unfortunately, but that shouldn’t matter, Just enjoy what you do.
An expert/master should be able to teach their craft also.
As someone who has recently completed a master of music, I can honestly say i still have so much to learn and would never class myself a ‘master’ because there is always so much to learn.
I hope this helps and keep practicing