Lewis & Short

prōmissĭo, ōnis f. [promitto].

  1. I. In gen., a promising, promise (class. only in sing.; prop. the act of promising; cf. promissum, the thing promised): provinciae, Cic. Att. 8, 9, 4: auxilii, id. Fam. 4, 13, 1: scelerum, id. Phil. 8, 3, 10; cf. Don. Ter. And. 2, 3, 27.
  2. II. In partic., rhet. t. t., a promising: tum promissio, si audierint, probaturos, Cic. de Or. 2, 83, 339; 3, 53, 205.