Definition of Plenum in Construction. A plenum is a space designated for air flow or can describe a space devoted to mechanical, electrical or sprinkler lines. The term plenum, could be a large section of ductwork, or the upper space above a ceiling. Any large space designated for any type of air flow or routing of systems, can be called a plenum. The space providing air is called a supply plenum and the space providing air return is referred to as a return plenum.