Lewis & Short

substĭtūtĭo, ōnis, f. [substituo, II.].

  1. I. In gen., a putting in the place of another, substitution, Arn. 3, p. 104.
  2. II. In partic., in jurid. lang., the appointment of a second or alternate heir, Gai. Inst. 2, § 174 sq.; Dig. 28, tit. 6.