Short term for surface active agent. Surfactants degrease and emulsify oils and fats and suspend soil, allowing them to be washed away. Surfactants are used in most forms of cleansers and many are considered gentle and effective for most skin types. There are several types of surfactants that can be drying and sensitizing for skin. When those are the main ingredients in a facial cleanser, body wash, or shampoo, they should be avoided; the most common among drying, sensitizing surfactants is sodium lauryl sulfate. The similar-sounding ingredient sodium laureth sulfate is fine.
References for this information:
International Journal of Toxicology, July 2010, pages 151S-161S
Journal of Cosmetic Science, March 2009, pages 143-151
Journal of the American Academy of Dermatology, January 2005, pages 125-132
International Journal of Cosmetic Science, June 1999, pages 371-382