Is it ensure, insure, or assure?
late 14c., from Latin praepositionem (nominative praepositio) "a putting before, a prefixing," noun of action from past participle stem of praeponere "put before," from prae "before" (see pre-) + ponere "put, set, place" (see position (n.)). In grammatical use, a loan-translation of Greek prothesis, literally "a setting before."
A part of speech that indicates the relationship, often spatial, of one word to another. For example, “She paused at the gate”; “This tomato is ripe for picking”; and “They talked the matter over head to head.” Some common prepositions are at, by, for, from, in, into, on, to, and with.