Source : Webster's Revised Unabridged Dictionary (1913)

Surface tension \Sur"face ten"sion\ (Physics)
   That property, due to molecular forces, which exists in the
   surface film of all liquids and tends to bring the contained
   volume into a form having the least superficial area. The
   thickness of this film, amounting to less than a thousandth
   of a millimeter, is considered to equal the radius of the
   sphere of molecular action, that is, the greatest distance at
   which there is cohesion between two particles. Particles
   lying below this film, being equally acted on from all sides,
   are in equilibrium as to forces of cohesion, but those in the
   film are on the whole attracted inward, and tension results.

Source : WordNet®

surface tension
     n : a phenomenon at the surface of a liquid caused by
         intermolecular forces