Penname

A Penname is a name that one writes under. It is similar to a username in that it is not normally the writer's actual name, but instead another name that they go under.

On fanfiction archives, one's penname is the same as their username, and you can usually tell the quality of a writer by their penname, as "Leggy_rox_my_sox23" is probably not going to have as good a story as someone with an original penname, or at least no numbers, unnecessary xs, dashes, and underscores.