What is an "unlawful detainer"?

An "unlawful detainer" refers specifically to a legal action for eviction—a situation in which a landlord seeks to remove a tenant from a rental property due to the tenant's failure to adhere to lease terms, often non-payment of rent or violating other stipulations of the rental agreement. This action is typically initiated after the landlord has provided a formal notice to the tenant to vacate the premises but the tenant remains in occupation despite the notice.

In the context of landlord-tenant law, the unlawful detainer process allows the landlord to seek a court order to regain possession of the property. It is a key step in the eviction process where the court assesses the validity of the landlord's claim and decides whether the tenant must leave the premises.

Other answer choices relate to aspects of rental agreements or tenant notifications but do not encapsulate the essence of an unlawful detainer. Specifically, modifying a rental agreement or providing a notice to vacate does not directly describe the legal action or the judicial process involved in evicting a tenant. Likewise, a type of rental lease expiration does not specifically pertain to the court-driven aspect of unlawful detainer actions. Therefore, the designation of unlawful detainer as a legal action for eviction is accurate and comprehensive.