TERTIUS INTERVENIENS, civil law. One, who claiming an interest in the subject or thing in dispute in action between other parties, asserts his right to act with the plaintiff, to be joined with him, and to recover the matter in dispute because he has an interest in it or to join the defendant, and with him, oppose the interest of the plaintiff, which it is his interest to defeat. He differs from the intervener or he who interpleads in equity. 4 Bouv. Inst. n. 3819, note.