http://hackage.haskell.org/package/base-4.6.0.1/docs/Data-Mo...
Methods
mempty
mappend
mconcat
Haskell people like their mathy terms. Mathy terms aren't universally unambigious ("group"? "ring"? "field"?), but they are mostly unambiguous within math. Haskell people tend to pretend Haskell is the same as math, ignoring the programming part of its heritage.In particular, it's easy to define a reverse monoid for any (non-commutative) monoid such that append becomes prepend. It's easy to construct monoids which have different spacial properties like Diagrams' "stacking" monoid (they have many others, too, see this entire paper http://www.cis.upenn.edu/~byorgey/pub/monoid-pearl.pdf). It's also easy to construct monoids which don't have any spatial sense at all like set union.
What is '+' ? Addition? Modular addition? Logical OR? Concatenaton? Sometimes, any of these.
There where always be more concepts than distinct labels, since the space of concept is exponential combination (power set) of words.
I don't have the perfect answer, but append certainly seems like choosing a specific concept , rather than trying to come up with a more general name.