1. If a substance is dissolved in two immiscible liquids standing in contact with each other, the substance will partition or distribute itself between them in a constant ratio, called the partition coefficient. The value of this constant is dependent on the temperature, and on the identities of the solute and the solvents. (The number of solute molecules in solvent A divided by the number of solute molecules in solvent B is a constant.)

2. The ratio of the concentration (by weight) of an element (e.g. Ti) in a crystallizing mineral to its concentration in the magma. For example, k_Ti = [Ti]_min./[Ti]_magma, where k_Ti is the partition coefficient for Ti, and [Ti]_min. and [Ti]_magma are the concentrations of Ti in the mineral and magma respectively. The value of k is dependent on temperature, pressure and the composition of the crystallizing mineral and magma.