the act or process of transpiring or excreting in the form of vapor; exhalation, as through the skin or other membranes of the body; as, pulmonary transpiration, or the excretion of aqueous vapor from the lungs. perspiration is a form of transpiration.-the evaporation of water, or exhalation of aqueous vapor, from cells and masses of tissue.-the passing of gases through fine tubes, porous substances, or the like; as, transpiration through membranes.