Sucker.

For those who've wondered just what a sucker looks like, gawk at this photo. This is a tomato plant sucker. As you can see, it's a new stem growing between two pre-existing, and more established, stems. Some people like to pinch them back. Others don't. But before you even have to make that decision, here's what you need to look for. This is a determinate variety of tomato, so I think I'll keep the suckers on and see what happens.