kernel has 14 distinct synonyms spread across 2 of its 3 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 inner and usually edible part of a seed or grain or nut or fruit stone
“black walnut kernels are difficult to get out of the shell”
the choicest or most essential or most vital part of some idea or experience
“the gist of the prosecutor's argument”
Synonyms, antonyms and the broader and narrower relations for kernel come from Princeton University’s WordNet. Full sources and licences.