An opposing force; right to left, dark to light.
From the Greek (via Latin) ἀντί meaning "against" and θέσις meaning "position."
As LJ doesn't usually take kindly to the Greek alphabet, the classical pronunciation is "antí" meaning "against" and "tithenai" or "thesis" meaning "position."
"To the church-going fundamentalists protesting equal rights for those of other religions and sexual orientations Janey shouted, "you are the very antithesis of your own God's ideals!"
Love Tahlia, writing an essay (if by "writing" she means "procrastinating on" and if by "essay" she means "everything, as usual.")