Definition of surfactant

Surface active agent - a substance that reduces the surface tension of a liquid by enriching at its surface. As a consequence, it also reduces the interfacial strain at the contact between this liquid and another liquid or a solid.

Ref: Kosswig, K. & Stache, H. (eds.): Die Tenside. Hanser-Verlag, 1993, 527 pp. (in German)