What is the meaning of walk-in?

noun

• Person who walks in without having an appointment.
  • usage: "the emergency room was overrun with walk-ins"
• An operative who initiates his own defection (usually to a hostile country) for political asylum.
• An assured victory (especially in an election.
  • synonyms: waltz
• A small room large enough to admit entrance.

adjective

• Of e. G. Closets or refrigerators. ( extending very far enough back to allow a person to enter)
  • usage: "a deep walk-in refrigerator"; "walk-in closets"