substituend
/səbˈstɪtjʊənd/ /səbˈstɪtʃʊənd/Chiefly Linguistics and Logic
noun
A thing that can be put in place of something else; a word, expression, etc., that may substitute for another.
Origin
1930s; earliest use found in W. V. Quine (1908–2000), philosopher. From classical Latin substituendus, gerundive of substituere substitute.