A syncline or synclinorium (Greek συγκλίνειν synklinein, tend to each other ') is the downward part of a fold ( well), which was created by combining compression of rocks under lateral pressure, the youngest layers are in the well core. Is the age sequence ( stratigraphy ) is not known, one speaks of a synform. It is the opposite of an anticline or antiform (saddle). It can thus also be a synformale anticline. Depending on the size of the fold of the syncline may be manifested as concave indentation of the earth's surface or the depth line which can follow the line of maximum depth along a valley.

The Geosynklinaltheorie that explains the formation and change of the earth's crust by the formation of large synclines beneath the seas, was referred to the development of the theory of plate tectonics predominant explanation of mountain building, also orogeny.