The Collaborative International Dictionary of English v.0.48:

Affreightment \Af*freight"ment\, n. [Cf. F. affr['e]tement.]
   The act of hiring, or the contract for the use of, a vessel,
   or some part of it, to convey cargo.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

AFFREIGHTMENT, Com. law. The contract by which a vessel or the use of it, is
let out to hire. See Freight; General ship.