substituend

noun2

1. •
   A substitute; something that substitutes for another.

   • Each substituend SN, it seems, would amount to the same thing as its corresponding UN in SOME not very strained context. — 1972
2. •
   A substituendum; something to be substituted or replaced.

   • […]but since the substitute y is not treated like the substituend vowel, the option will not prevail. — 1989