neck has 3 distinct synonyms spread across 3 of its 6 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.
the part of an organism (human or animal) that connects the head to the rest of the body
“he admired her long graceful neck”
a narrow elongated projecting strip of land
Synonyms, antonyms and the broader and narrower relations for neck come from Princeton University’s WordNet. Full sources and licences.
a cut of meat from the neck of an animal
a narrow part of an artifact that resembles a neck in position or form
“the banjo had a long neck”
an opening in a garment for the neck of the wearer; a part of the garment near the wearer's neck