Cal. Civ. Proc. Code § 426.10

As used in this article:

(a) “Complaint” means a complaint or cross-complaint.

(b) “Plaintiff” means a person who files a complaint or cross-complaint.

(c) “Related cause of action” means a cause of action which arises out of the same transaction, occurrence, or series of transactions or occurrences as the cause of action which the plaintiff alleges in his complaint.