Is it ensure, insure, or assure?
late 14c., "appointment of a subordinate or successor," from Middle French substitution, from Late Latin substitutionem (nominative substitutio) "a putting in place of another," from past participle stem of Latin substituere "put in place of another, place under or next to," from sub "under" (see sub-) + statuere "set up," from PIE root *sta- "to stand," with derivatives meaning "place or thing that is standing" (see stet).
early 15c. in transitive sense, 1888 as intransitive, from Latin substitutus, past participle of substituere (see substitution). Related: Substituted; substituting.
"one who acts in place of another," early 15c., from Old French substitute and directly from Latin substitutus, past participle of substituere (see substitution). Team sports sense is from 1849.
substitution sub·sti·tu·tion (sŭb'stĭ-tōō'shən, -tyōō'-)
The replacement of an atom or group of atoms in a compound by another atom or group of atoms.
An unconscious defense mechanism by which the unacceptable or unattainable is replaced by something more acceptable or attainable.