tendency has 5 distinct synonyms spread across 4 of its 4 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.
an attitude of mind especially one that favors one alternative over others
“he had an inclination to give up too easily”
Synonyms, antonyms and the broader and narrower relations for tendency come from Princeton University’s WordNet. Full sources and licences.
an inclination to do something
“he felt leanings toward frivolity”
a characteristic likelihood of or natural disposition toward a certain condition or character or effect
“the alkaline inclination of the local waters”