Real talk, “master” refers to a “master copy” from which new branches are can be made from. If one of these branches becomes “better” than the current master copy, it gets “merged” into the master copy effectively replacing it as the new master copy (sometimes totally, sometimes partially).

Edit: Pretty sure basically everyone here knows this, but I figured it could not hurt to put the explanation here.

I use master and apprentice. Always two there are, no more, no less.

Why not call it trunk and make it all tree themed

Treat branches like Chinese dynasties. The mainline branch is the one having the mandate of heaven.

I dislike master because main is shorter and faster to type