I know pixel might be right, but pixel means something different, especially if a "2D voxel" isn't exactly one pixel (it isn't, in my game).
I've also heard "texel," but that seems to refer to a repeating texture, which this isn't. I guess I'm just getting hung up on words, but is there a proper term for this?
(For an example of the thing I'm referring to, look at Terraria.)
Pixel.
The term voxel is short for "volume pixel" or "volume picture element." Consequently a voxel without the "volume" bit is just a regular pixel.
The term for gameplay structural unit in a 2D game that can be deformed, damaged or destroyed in the course of game play (as in Terraria and games of its ilk) would most commonly be called a "tile," although you're likely to see more variety in terminology there.
I'd venture to guess what has you confused is that you're assuming a voxel represents a unit-of-destruction in 3D (because it does, or at least appears to, in Minecraft). This is not, however, necessarily true. It's just a particular pattern that has been popularized by the Minecraft-like genre of games right now (especially since historically complex voxel models have been CPU-intensive and it was a useful optimization to keep them one-to-one).
However, just like a pixel need not represent a unit of destruction in 2D, a voxel need not do the same in 3D.