Back in the day, game books referred to the players. Not the players and their characters, just the players. It was a synonymous term with "player characters."
[Editing later: This is clearly a bit inaccurate. Both were used from extremely early on . . . but it wasn't a rigid distinction of terms, where you'd have to use player to me the actual person, and character for the alter-ego, not ever use one term when you meant the other.]
Later, it changed to "players" for the actual people playing the game, and "characters" for their imaginary alter-ego / paper men in the game. You go and say something like "One of the players died in my game last night" and you get this mock-serious reply of "Oh, your players die? Your games sure are lethal! Hahahah" etc.
When did that happen?
1974's D&D, Book I: Men & Magic has an "example record of a character" named Xylarthen, followed by the line "This supposed player would have progressed faster as a Cleric [. . . ]"
In Strategic Review 2/2, last issue before The Dragon #1, Gary Gygax wrote an article that was reprinted in volume I of the Best Of The Dragon issues, called "D&D Is Only As Good As The DM." In it we get this gem of a quote:
"Second, absolute disinterest must be exercised by the Dungeonmaster, and if a favorite player stupidly puts himself into a situation where he is about to be killed, let the dice tell the story and KILL him."
Good advice, certainly, but nowadays we think of player as the person not the character.
Even then you had people saying "character" not "player" but the terms would get mixed together pretty frequently.
But after a while, that stopped.
"Player" started to mean the person, and "character" the imaginary alter-ego. I'm curious about when that happened. I don't have time to dig down and research it, but I've kind of mulled that one over for a while. When was it clear you couldn't co-mingle the terms usefully?
Flame Retardant: I am not interested in this as a "bad thing" or a "good thing" but just from a more historical perspective . . . when did it make that interesting linguistic shift?