Surface tension is a contractive tendency of the surface of a liquid that permits it to resist an external force. It is revealed, for instance, in the floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids.

Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent-but when referring to energy per unit of area, people use the term surface energy-which is a more general term in the sense that it applies also to solids and not just liquids.