An eraser is the small piece of rubber — or a similar soft stuff — that you rub on paper to take away pencil, and sometimes pen. In British English people often just say rubber for the same thing; in American English eraser is the ordinary classroom word, and rubber can make people laugh because it also means a condom. On a pencil it sits at the end; on a desk it is a separate block. Some are for pencil only; ink erasers and whiteboard erasers are named for what they wipe. It is a homely, everyday noun — nothing formal about it. Though actually, it only names the tool. The act of rubbing out, or the empty space left behind, is erasure, a different word.