I think it's called a "baffle". The original one, which was probably between 12-15 years old, held up well but eventually started coming apart such that drops of crud could spray up and out of the disposal on occasions when the motor was on. I was expecting a tedious and potentially expensive repair that would involve removing the unit from the sink. But it turns out, at least with the brand I have (Insink--or), that the baffle is designed to be easily replaced. All you have to do is reach in and yank out the old one (nothing holds it in but friction), then plug the new one back in its place. Done.
The old baffle had all kind of nasty brown crud on the bottom of it and there are horrors inside the unit of course, but it's working fine. I don't often use the unit and that no doubt accounts for its longevity. The best part, apart from how easy the fix was, was how cheap it was - the baffles themselves cost me less than $10 each online.
