An interloper is someone who comes into a place, a group, or a business where they should not be — and often where they are not wanted. The feeling is a little sharp: not a guest, and not always a thief; someone who has pushed in. Long ago it meant a trader who broke into another company’s private market. Today people use it for a stranger at a family table, a person who walks into a closed meeting, or even an animal in a garden. It is a written, slightly old-fashioned word; in speech many people would say intruder, or simply someone who doesn’t belong. Though actually, it does not have to be a crime — the hurt is “you are not one of us,” not only “you broke a law.”