An interloper is someone who shows up where they don't belong or weren't invited, often interfering with what others are doing. When a new kid pushes their way into an established friend group without being welcomed, they're acting like an interloper. When someone barges into a private conversation they weren't part of, they're interloping.

The word carries a sense of unwelcome intrusion. An interloper inserts themselves into situations, spaces, or activities where they have no real right to be, deliberately pushing past boundaries.

You might call someone an interloper if they crash a party uninvited, or if they try to take over a project they had nothing to do with starting. The word suggests not just being an outsider, but an outsider who refuses to recognize they're out of place. While sometimes people become interlopers by honest mistake, the word usually implies pushiness or disregard for boundaries that should have been obvious.