An unwelcome visitor is someone who is not wanted or invited, and can be described in a variety of ways. The 8-letter term for this is 'INTRUDER, ' which refers to someone who enters a place without permission. A 10-letter term is 'TRESPASSER, ' which is someone who enters a property without the owner's consent. Another 10-letter term is 'Grim Reaper, ' which is a metaphor for death, as it is an unwelcome visitor that no one wants to see. Finally, the 15-letter term is 'persona non grata, ' which is a Latin phrase meaning 'an unwelcome person.'