Outside writing is what you call it when outside influences, like editors, influence the way a story is written, not the natural flow of the story.
For instance, in the Superman titles awhile back.
"Lois and Clark need to break up!"
Then, a few issues later, "No wait, they have to get married tomorrow!"
Silly nonsense like that makes for bad comic books often....but not ALWAYS...
For instance, Morrison's JLA was big-time outside writing, Morrison really did not have an "inside" story reason why the "Big Seven" should form a team together, but the comic was excellent anyways.
This is not just a comic term, it also applies to other serial medium, like television. "Character X loves Character Y sooo much. But Character X's contract is up, and is leaving the show. So now Character X hates Character Y."