Lewis & Short

assessor (ads-), ōris, m. [assideo], he that sits by one, an assessor, aid: Lacedaemonii regibus suis augurem adsessorem dederunt, Cic. Div. 1, 43, 95.
In judic. lang., the assistant of a judge, assessor (cf. Zimm. Rechtsgesch. 3, p. 21 sq.; Hugo, Rechtsgesch. p. 685), Dig. 1, 22; Suet. Galb. 14; Sen. Tranq. 1, 3.