substitution

英[sʌbstɪ'tjuːʃn] 美[,sʌbstə'tjʊʃən]

1. an event in which one thing is substituted for another;

"the replacement of lost blood by a transfusion of donor blood"

2. the act of putting one thing or person in the place of another: "he sent Smith in for Jones but the substitution came too late to help"