Nexum

Lat. In Roman law. In ancient times the nexum seems to have been a species of formal contract, involving a loan of money, and attended with peculiar consequences, solemnized with the "copper and balance." Later, it appears to have been used as a general term for any contract struck with those ceremonies, and hence to have included the special form of conveyance called "mancipatio." In a general sense It means the obligation or bond betwcen contracting parties.

See Maine, Anc. Law, 305, et seq.; Hadl. Rom. Law, 247.

In Roman law, this word expressed the tie r obligation involved in the old conveyance by mancipatio; and came latterly to be used interchangeably with (but less frequently than) the word "obligatio" itself.

Brown.