Regardless of how it has been used, I tend to think of the word powergaming as being a subset of problem-players, and not of optimization. While optimization is commonly used with respect to power (i.e. optimizing a character’s power), it is far from the only use of optimization. Moreover, powergaming as a term is typically a pejorative in the RPG community, wherein the problem being described is a player’s devotion to mechanical power, at the detriment of the game as a whole.
In other words, I think the common use described by @MrLemon is an inaccurate use.
Unfortunately, that does not lend itself to a happy solution; making powergaming a synonym of problem-players causes too many people to miss optimization, and as @BESW says, if we don’t make it a synonym of something it will just keep getting recreated. The ideal would be a sort of “did you mean..?” question with both options, but the system doesn’t support that (and I don’t think it would be a particularly good addition in general).
I just think that powergaming and optimization are not synonymous, and establishing the tags as synonyms just seems wrong. Does anyone know if there is any precedent on other SEs about this sort of issue, where you have a tag that’s too frequently used to try to keep deleting it, but is used in two very different senses that don’t lend themselves to a single synonym?