walk-in (Adjective) — (of e.g. closets or refrigerators) extending very far enough back to allow a person to enter. ex. "a deep walk-in refrigerator" ex. "walk-in closets"
walk-in (Noun) — Person who walks in without having an appointment. ex. "the emergency room was overrun with walk-ins"
walk-in (Noun) — An operative who initiates his own defection (usually to a hostile country) for political asylum.
walk-in (Noun) — An assured victory (especially in an election).
walk-in (Noun) — A small room large enough to admit entrance.
walk-in (Verb) — Enter by walking. ex. "She walks in at all hours, as if she lived here"