walk-in has 1 distinct synonym spread across 1 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
person who walks in without having an appointment
“the emergency room was overrun with walk-ins”
an operative who initiates his own defection (usually to a hostile country) for political asylum
1 of the 5 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for walk-in.
Synonyms, antonyms and the broader and narrower relations for walk-in come from Princeton University’s WordNet. Full sources and licences.
a small room large enough to admit entrance