A more thorough explanation:

Definition: Detainer refers to the act of holding or keeping something in one's custody. It can also refer to the confinement of a person in custody or a writ authorizing a prison official to continue holding a prisoner in custody.

For example, if a tenant continues to occupy a property after their lease has ended and the landlord has demanded possession, this is known as unlawful detainer. Another example is when a person is arrested and held in custody, they may be subject to a detainer, which allows the prison officials to continue holding them.

Detainer can also refer to a person who detains someone or something. For instance, a security guard who detains a shoplifter until the police arrive is considered a detainer.