Antithesis is word generally used to mean 'opposite'. As a figure of speech, it is grouped in AWE as a figure of sound patterning. Here it means balancing two units, usually clauses, with contrasting meanings. The opening of Dickens' novel A Tale of Two Cities is a good example: "It was the best of times, it was the worst of times." Here an obvious opposite ('best' and 'worst') is emphasised by the parallel structure: the clause is repeated with only one word changed in order to reverse the idea. Alexander Pope said, in An Essay on Criticism, "To err is human, to forgive divine" using antithesis to bring out the difference between human fallibility and the goodness of God.
Antithesis is also a term in logic and structured argument. OED definmes it as "a proposition opposed to a thesis; a counter-thesis or -proposition." The Dialectic is a term often used to mean the system of studying History, and developed to argue about economics and politics, taught by Karl Marx, and derived by him from the German philosopher Hegel.)
(For a note about the form of antithesesis and its plural, see antithes-.)