basal has 5 distinct synonyms spread across 3 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.
especially of leaves; located at the base of a plant or stem; especially arising directly from the root or rootstock or a root-like stem
“basal placentation”
of or being the essential or basic part
“an elementary need for love and nurturing”
serving as or forming a base
“the painter applied a base coat followed by two finishing coats”
Synonyms, antonyms and the broader and narrower relations for basal come from Princeton University’s WordNet. Full sources and licences.