Lewis & Short

păcĭo, ōnis, f., a contract, covenant (ante-class. for pactio): pacionem antiqui dicebant, quam nunc pactionem dicimus: unde et pacisci adhuc et paco in usu remanet, Fest. p. 250 Müll. (perh. too, ap. Fest. s. v. nuptias, p. 170, instead of ratio we should read pacio, acc. to the conject. of Dac. on the preced. passage).