Is it ensure, insure, or assure?
mid-15c., "relating to or marking boundaries," from Latin terminalis "pertaining to a boundary or end, final," from terminus "end, boundary line" (see terminus). Meaning "fatal" (terminal illness) is first recorded 1891. Sense of "situated at the extreme end of something" is from 1805. Slang meaning "extreme" first recorded 1983.
"end point of a railway line," 1888, from terminal (adj.); sense of "device for communicating with a computer" is first recorded 1954.
terminal ter·mi·nal (tûr'mə-nəl)
Of, relating to, situated at, or forming a limit, a boundary, an extremity, or an end.
Of, relating to, occurring at, or being the end of a section or series; final.
Causing, ending in, or approaching death; fatal.