mutual relation of two or more things, parts, etc.: Studies find a positive correlation between severity of illness and nutritional status of the patients.
Statistics. the degree to which two or more attributes or measurements on the same group of elements show a tendency to vary together.
Physiology the interdependence or reciprocal relations of organs or functions.
Geology the demonstrable equivalence, in age or lithology, of two or more stratigraphic units, as formations or members of such.