Definition - What does Unlawful Detainer mean?
An unlawful detainer refers to a person or entity that is in possession of a piece of real property without proper legal authority. Alternatively, the term unlawful detainer can also be used to describe a person who is unlawfully impeding a person's ability to travel freely.
Justipedia explains Unlawful Detainer
Within the real property sense of the word, unlawful detainer is usually committed by a person who is lawfully required to leave where they are currently living due to some illegal act committed on the part of the person living in the space. For example, if a person does not pay their rent, then they are an unlawful detainer until they either pay their rent or leave the place they are living. In the other sense of the word, a police officer would be an unlawful detainer if he or she arrested somebody without having probably case to do so.