What does "correlation" mean?

Correlation: Reciprocal relation; corresponding similarity or parallelism of relation or law; capacity of being converted into, or of giving place to, one another, under certain conditions; as, the correlation of forces, or of zymotic diseases. Correlation of energy, the relation to one another of different forms of energy; -- usually having some reference to the principle of conservation of energy. See Conservation of energy, under Conservation. -- Correlation of forces, the relation between the forces which matter, endowed with various forms of energy, may exert.