Substituend

Each substituend SN, it seems, would amount to the same thing as its corresponding UN in SOME not very strained context.

Source: wiktionary

Note that a variable is a substituend of a variable only in a vacuous sense , and so we shall be ignoring them while speaking of the substituends of a given variable.

Source: wiktionary

The pragmatic process of substitution requires some given context to provide an appropriate substituend for a metavariable projected by a pronoun.

Source: wiktionary

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

Source: wiktionary