antithesis – the exact opposite
(Pronounced an-tith-uh-sis)
To remember the definition of the word antithesis, use the following mnemonic:
The ant with a temper insisted (antithesis) on doing the exact opposite of all the other ants.

Examples of antithesis in a sentence
He was the antithesis of her ideal employee.
Their latest album was the antithesis of their previous work.
Despite being the antithesis of one another, they became very good friends.