sur·fac·tant 
(sər-făktənt, sûrfăk′-)
(sər-făktənt, sûrfăk′-)n.
1. A surface-active substance.
2. A substance composed of lipoprotein that is secreted by the alveolar cells of the lung and serves to maintain the stability of pulmonary tissue by reducing the surface tension of fluids that coat the lung.
[SURF(ACE)-ACT(IVE) + A(GE)NT.]