Dictionary.com

noun (used with a singular verb)
1. the art or practice of disputation or controversy:
a master of polemics.
2. the branch of theology dealing with the history or conduct of ecclesiastical disputation and controversy.

Etymonline.com

1630s, “controversial argument or discussion,” from French polémique (16c./17c.), noun use of adjective meaning “disputatious, controversial” (see polemic (adj.)).

Discussion/Explanation

From the definition, you may start getting the idea that polemics is akin to apologetics. While they are related in the defense that they give, there is a significant difference.

Apologetics is focused outward whereas polemics is focused inward.

“In polemics, unlike apologetics, we are making an inward defense of our faith, rooting out false teachings and false teachers within the body of the church. A polemicist duty is to point out false teaching and teachers while warning others in the body of Christ about said teaching or teacher.”   — Richard Haas

Polemics is effectively an internal or within the church defense. It is the act of engaging and dealing with those who would rise up from within to lead others astray. Call such people the proverbial “wolf in sheep’s clothing” as the target of the polemicist. This is yet another hat particularly emphasized for leaders of the church but one that should be exercised by believers in general.