Русско-англо-русский юридический словарь

In civil procedure, a party's claim is a counterclaim if the defending party has previously (in the present action) made a claim against the claiming party. (Wikipedia)

countersuit: an opposing lawsuit

especially : a claim asserted by an opposing party in a lawsuit (such as a defendant) against the party bringing the original lawsuit (such as a plaintiff)

//decided to file a countersuit against her accuser (Merriam-Webster Dictionary)

If you take someone to court, they may respond by filing a countersuit against you. If a tenant sues their landlord for unfairly evicting them, the landlord might file a countersuit claiming months of unpaid rent. In law, a countersuit is sometimes called a counterclaim. (Vocabulary.com)

It’s important to note that, in many jurisdictions of the United States, the words countersuit and counterclaim are synonymous. However, in some jurisdictions, a counterclaim is merely an answer or denial of the plaintiff’s (creditor’s) allegation. Therefore, if you are told a counterclaim has been filed, clarify its meaning immediately. (Countersuit and Consequence - Law Offices of Gary A. Bemis)