Possessing only length or width, but not both. An ideal, geometric line, one that is infinitely long but has zero width, is a one-dimensional object.

In a literary context, a one-dimensional character is one possessing ambitions but no clear motives for them, e.g., a homicidal villain who wants to take over the world but doesn't seem to know what he's going to do with it afterwards. A derisive term, and one to be avoided by aspiring writers.