Lewis & Short

interventor, ōris, m. [intervenio].

  1. I. One who comes in, a visitor: vacuus ab interventoribus dies, Cic. Fat. 1, 2.
  2. II. A surety, bondsman: si filius fidejussor, vel quasi interventor acceptus sit, Dig. 15, 1, 3.
  3. III. An intercessor, mediator, Lampr. Comm. 4.