condition has 15 distinct synonyms spread across 8 of its 13 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 assumption on which rests the validity or effect of something else
a mode of being or form of existence of a person or thing
“the human condition”
information that should be kept in mind when making a decision
“another consideration is the time it would take”
the state of (good) health (especially in the phrases `in condition' or `in shape' or `out of condition' or `out of shape')
an illness, disease, or other medical problem
“a heart condition”
the procedure that is varied in order to estimate a variable's effect by comparison with a control condition
develop (a child's or animal's) behavior by instruction and practice; especially to teach self-control
“Parents must discipline their children”
put into a better state
“he conditions old cars”
apply conditioner to in order to make smooth and shiny
“I condition my hair after washing it”
Synonyms, antonyms and the broader and narrower relations for condition come from Princeton University’s WordNet. Full sources and licences.