The money paid by a person for the transportation of goods.
From time to time, the law gives specific names to the amount of money paid pursuant to contract for a particular service such as for the lease of real estate, rent.
And so it is, particular to the contract of affreightment: the money or cash consideration is called freight.
References and Further Reading:
Scrutton on Charterparties and Bills of Lading, 20th Edition, London: Sweet & Maxwell, 1996.