from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
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.