2 player game:
"I can see that no matter what I do, I am going to lose this game and you, my single opponent, are going to win. As ranking is determined solely by wins and losses (and not by score during the game) I will concede this game so as to not waste either my time nor yours. Congratulations on the win."
This is conceding, and is generally acceptable.
Some people want the definite victory, and will request the game be played out. This does waste the player's time, as it could be avoided. However, it only wastes future time, and players know the time will be wasted.
2+ player game:
"The table, as a whole, is not having fun. While we have some time invested in this game, the amount of time required to finish it is not worth the prestige nor ranking we as a group will gain. Let's cut our losses now."
This is proposing to abandon the game collectively, and depends on the group at the table. Again, generally acceptable, but sometimes isn't (especially when the purpose is "I can't win, so if I can get everyone to quit I'll not have a loss, and I'll threaten poor sportsmanship if you don't agree.").
3+ player game:
"I can see that no matter what I do, I am not going to win. Ranking is based on how all players finish, but since I can't be first, I don't care how I finish. I also don't care about the time others have already invested in this game, and how the game mechanics change if a player drops out. I'm leaving."
This is quitting a game in progress, unacceptable, and is penalized by lowering reputation automatically. Some people have this attitude and try to avoid the penalty by proposing to abandon the game. It still falls under unacceptable behavior.
Yes, all actions are ending the game before the game's end conditions are met. However, there are differences in intent and how it affects others, and those differences are important.