Definition of PLENUM. 1. a: a space or all space every part of which is full of matter b: an air-filled space in a structure; especially: one that receives air from a blower for distribution (as in a ventilation system). 2.. a: a space or all space every part of which is full of matter b: an air-filled space in a structure; especially: one that receives air from a blower for distribution (as in a ventilation system).