Stylolites (Greek: stylos, pillar; lithos, stone) are serrated surfaces within a rock mass at which mineral material has been removed by pressure dissolution, in a deformation process that decreases the total volume of rock. Minerals which are insoluble in water, such as clays, pyrite and oxides, as well as insoluble organic matter, remain within the stylolites and make them visible. Sometimes host rocks contain no insoluble minerals, in which case stylolites can be recognized by change in texture of the rock. They occur most commonly in homogeneous rocks, carbonates, cherts, sandstones, but they can be found in certain igneous rocks and ice. Their size vary from microscopic contacts between two grains (microstylolites) to large structures up to 20 m in length and up to 10 m in amplitude in ice. Stylolites usually form parallel to bedding, because of overburden pressure, but they can be oblique or even perpendicular to bedding, as a result of tectonic activity.
In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress within the same rock or their complete removal from the rock within the fluid. It is an example of diffusive mass transfer. Stylolites are formed by this process.
Stylolites can be classified according to their geometry or their orientation and relationship to bedding.
Park and Schot (1968) recognized six different geometries in stylolites:
A stylolite is not a structural fracture, although they have been described as a form of 'anticrack', with the sides moving together rather than apart. Proof exists in the form of fossiliferous limestone where fossils are crosscut by a stylolite and only one half still exists; the other half has been dissolved away. Rye & Bradbury (1988)  investigated 13/12C and 18/16O stable isotope systematics in limestone on either side of a stylolite plane and found differences confirming different degrees of fluid-rock interaction.
In order for a stylolite to develop, a solution into which minerals can dissolve needs to be present, along with a pore network through which dissolved solids can migrate by advection or diffusion from the developing stylolite. Stylolite development can be improved with porosity, as it localizes stress on grains, increasing the stress there. Therefore, it is suggested that bedding-parallel stylolites form in areas of high porosity, and most of the transverse stylolites form along preexisting fractures.
Stylolites are significant in several fields. In petrology, stylolites are important because they alter rock fabrics and dissolve solids that precipitate as cement. In stratigraphy, weathering of stylolites generates apparent bedding in many stratigraphic sections and loss of material along stylolites can have a result similar to erosion, with significant stratigraphic thinning. In hydrology, stylolites prevent fluid flow and, in other settings, serve for fluid flow. Also, stylolites are indicators of compressive stress in tectonic studies, and development of transverse stylolites contributes to crustal shortening parallel to the direction of their column.