Русско-англо-русский юридический словарь

A defendant who has a claim against the plaintiff can bring what is known as a counterclaim, and at that point the defendant becomes what is known as the "counter-plaintiff" with respect to the counterclaim against the plaintiff (who is now also known as the "counter-defendant"). (Lawyers.com)

“claim” includes a counter-claim and, accordingly, “claimant” includes a counter-claimant and “defendant” includes a defendant to a counter-claim; (UK Criminal Justice and Courts Act 2015)