In materials science and biology, capillary condensation is the "process by which multilayer adsorption from the vapor [phase] into a porous medium proceeds to the point at which pore spaces become filled with condensed liquid from the vapor [phase]." The unique aspect of capillary condensation is that vapor condensation occurs below the saturation vapor pressure, , of the pure liquid.  This result is due to an increased number of van der Waals interactions between vapor phase molecules inside the confined space of a capillary.  Once condensation has occurred, a meniscus immediately forms at the liquid-vapor interface which allows for equilibrium below the saturation vapor pressure.  Meniscus formation is dependent on the surface tension of the liquid and the shape of the capillary, as shown by the Young-Laplace equation.  As with any liquid-vapor interface involving a meniscus, the Kelvin equation provides a relation for the difference between the equilibrium vapor pressure and the saturation vapor pressure.  A capillary does not necessarily have to be a tubular, closed shape, but can be any confined space with respect to its surroundings.
Capillary condensation is an important factor in both naturally-occurring and synthetic porous structures.  In these structures, scientists use the concept of capillary condensation to determine pore size distribution and surface area through adsorption isotherms.  Synthetic applications such as sintering of materials are also highly dependent on bridging effects resulting from capillary condensation.  In contrast to the advantages of capillary condensation, it can also cause many problems in materials science applications such as atomic-force microscopy and microelectromechanical systems.