sign has 24 distinct synonyms spread across 12 of its 20 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.
a public display of a message
“he posted signs in all the shop windows”
any nonverbal action or gesture that encodes a message
“signals from the boat suddenly stopped”
(astrology) one of 12 equal areas into which the zodiac is divided
(medicine) any objective evidence of the presence of a disorder or disease
“there were no signs of asphyxiation”
having an indicated pole (as the distinction between positive and negative electric charges)
“he got the polarity of the battery reversed”
an event that is experienced as indicating important things to come
“he hoped it was an augury”
a gesture that is part of a sign language
a fundamental linguistic unit linking a signifier to that which is signified
“The bond between the signifier and the signified is arbitrary”
a character indicating a relation between quantities
“don't forget the minus sign”
approve and express assent, responsibility, or obligation
“All parties ratified the peace treaty”
used of the language of the deaf
Synonyms, antonyms and the broader and narrower relations for sign come from Princeton University’s WordNet. Full sources and licences.
engage by written agreement
“They signed two new pitchers for the next season”
communicate silently and non-verbally by signals or signs
“He signed his disapproval with a dismissive hand gesture”
place signs, as along a road
“sign an intersection”
communicate in sign language
“I don't know how to sign, so I could not communicate with my deaf cousin”
make the sign of the cross over someone in order to call on God for protection; consecrate