posture has 12 distinct synonyms spread across 6 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 arrangement of the body and its limbs
“he assumed an attitude of surrender”
a rationalized mental attitude
capability in terms of personnel and materiel that affect the capacity to fight a war
“we faced an army of great strength”
behave affectedly or unnaturally in order to impress others
“Don't pay any attention to him--he is always posing to impress his peers!”
Synonyms, antonyms and the broader and narrower relations for posture come from Princeton University’s WordNet. Full sources and licences.