What is the thing that stops the sink drain called?

The thing that stops a sink from draining all water is called a sink plug or a drain plug. It's usually made out of rubber since rubber doesn't go rusty or doesn't eventually decompose in water so once bought, it can be used almost indefinitely since nothing can harm it other than people or heat or any other destructive factor.

It is called a plug that is what stops the water from draining or what ever is in the sink.