§ 426.10.Definitions

Section 426.10 sets the vocabulary for the compulsory cross-complaint rules that follow, most importantly § 426.30. It defines “complaint” to include a cross-complaint and “plaintiff” to mean whoever filed one, so the article's terms apply symmetrically whether the underlying claim came in as an original complaint or as a cross-complaint against someone else.

The definition that carries the most weight is “related cause of action” in subdivision (c): a cause of action arising out of the same transaction, occurrence, or series of transactions or occurrences as the cause the plaintiff has alleged. That phrase is the trigger for § 426.30's use-it-or-lose-it rule — a defendant who has a related cause of action against the plaintiff, in this sense, must raise it in a cross-complaint or risk losing the right to bring it later.

Because the definition turns on a factual relationship (same transaction or series of transactions) rather than a fixed list of claim types, whether a given counterclaim is “related” is a case-specific question, decided by how closely the facts of the two claims overlap.