1. (Law) One who deposes or testifies under oath; one who gives evidence; usually, one who testifies in writing. 2. (Gr. & Lat. Gram.) A deponent verb. Syn. -- Deponent, Affiant. These are legal terms describing a person who makes a written declaration under oath, with a view to establish certain facts. An affiant is one who makes an affidavit, or declaration under o...