IPA: /ˌsʌb.stɪˈtjuː.ʃən.əri/
KK: /sʌbstəˈtuːʃənɛri/
Relating to the act of replacing one thing with another.
The substitutionary principle in mathematics allows for one variable to be replaced with another.
Substitutionary → It is formed from "sub-" (meaning under or in place of) and "stituere" (meaning to place or set). The word refers to something that is placed in the place of another, often in a legal or formal context.
Think of something being 'placed in the place of' another — 'sub-' means under or in place of, and 'stituere' means to place.