Antonym and Contra

Antonym noun - A word that expresses a meaning opposed to the meaning of another word, in which case the two words are antonyms of each other.

Contra and antonym are semantically related In some cases you can use "Contra" instead a noun "Antonym".

Contra noun - A member of the guerrilla force that opposed a left-wing government in Nicaragua.

Antonym and contra are semantically related in something completely unlike topic. Sometimes you can use "Antonym" instead a noun "Contra".