Surfactant: A fluid secreted by the cells of the alveoli (the tiny air sacs in the lungs) that serves to reduce the surface tension of pulmonary fluids; surfactant contributes to the elastic properties of pulmonary tissue.

In more technical terms, a surfactant is a surface active agent, the best known example being the lung surfactant that is produced shortly before birth and makes the alveolar surfaces hydrophobic and prevents the lung from filling with water.