substitutio
From LSJ
substĭtūtĭo: ōnis, f. substituo, II..
I In gen., a putting in the place of another, substitution, Arn. 3, p. 104.—
II In partic., in jurid. lang., the appointment of a second or alternate heir, Gai. Inst. 2, § 174 sq.; Dig. 28, tit. 6.