Surface tension is a property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of the water. It is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water, such as razor blades and insects, to float on a water surface.

Surface tension is not a property of the liquid alone, but a property of the liquids interface with another medium. The surface of any liquid is an interface between that liquid and some other medium. For example, the top surface of a pond is an interface between the pond water and the air.

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, it is common to use the term surface energy, which is a more general term in the sense that it applies also to solids. In materials science, surface tension is used for either surface stress or surface energy.

Surface tension depends mainly upon the forces of attraction between the particles within the given liquid and also upon the gas, solid, or liquid in contact with it. The molecules in a drop of water, for example, attract each other weakly. Water molecules well inside the drop may be thought of as being attracted equally in all directions by the surrounding molecules. However, if surface molecules could be displaced slightly outward from the surface, they would be attracted back by the nearby molecules. The energy responsible for the phenomenon of surface tension may be thought of as approximately equivalent to the work or energy required to remove the surface layer of molecules in a unit area. Surface tension may be expressed, therefore, in units of energy (joules) per unit area (square meters) .

In summary, surface tension is a property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of the water. It is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than water, such as razor blades and insects, to float on a water surface.