Word for a software bug that occurs again after having fixed it?

What's the word for a software bug that reoccurs after being fixed?

I'm embarking on some test-driven development (TDD) for a project, and I want to use the right term but have forgotten the word.

It's not reversion, but I think it sounds similar.

Though I've used it many times I can't recollect the term right now, and Google isn't helping me.

Addition 1: The bug is imaginary in this case. It only reappears once, after I fixed, because of a new change I introduce. I'm not talking about something that that keeps on popping up.

Addition 2: The correct answer popped up 7 minutes after asking. The amount of bikeshedding the keeps on coming after that is hilarious.

Solution 1:

You're looking for "regression".

Solution 2:

Such bugs may simply be called recurring bugs. There are even papers on such bugs:

Previous research confirms the existence of recurring bug fixes in software systems...

Solution 3:

The bug was reintroduced by a subsequent change, resulting in a regression.