The reduction in the freezing point of a pure liquid when another substance is dissolved in it. It is a colligative property – i.e. the lowering of the freezing point is proportional to the number of dissolved particles (molecules or ions), and does not depend on their nature. It is given by Δ‎t = K_fC_m, where C_m is the molar concentration of dissolved solute and K_f is a constant (the cryoscopic constant) for the solvent used. Measurements of freezing-point depression (using a Beckmann thermometer) can be used for finding relative molecular masses of unknown substances.

http://web.lemoyne.edu/~giunta/raoult.html Raoult’s original paper