Seize  v. (-zing) 1 (often foll. By on, upon) take hold of forcibly or suddenly. 2  take possession of forcibly or by legal power. 3 affect suddenly (panic seized us). 4 (often foll. By on, upon) take advantage of (an opportunity etc.). 5 (often foll. By on, upon) comprehend quickly or clearly. 6 (also seise) (usu. Foll. By of) law put in possession of.  seized (or seised) of 1 possessing legally. 2 aware or informed of. Seize up 1 (of a mechanism) become jammed. 2 (of part of the body etc.) Become stiff. [french saisir]