readiness has 7 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.
the state of having been made ready or prepared for use or action (especially military action)
“putting them in readiness”
prompt willingness
“readiness to continue discussions”
(psychology) being temporarily ready to respond in a particular way
“the subjects' set led them to solve problems the familiar way and to overlook the simpler solution”
a natural effortlessness
“they conversed with great facility”
Synonyms, antonyms and the broader and narrower relations for readiness come from Princeton University’s WordNet. Full sources and licences.