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early 15c., from Medieval Latin internalis, from Latin internus "within, inward, internal," figuratively "domestic," expanded from pre-Latin *interos, *interus "on the inside, inward," from PIE *en-ter- (cf. Old Church Slavonic anter, Sanskrit antar "within, between," Old High German unter "between," and the "down" sense of Old English under); suffixed (comparative) form of *en "in" (see in). Meaning "of or pertaining to the domestic affairs of a country (e.g. internal revenue) is from 1795. Internal combustion first recorded 1884. Related: Internally.
internal in·ter·nal (ĭn-tûr'nəl)
Located, acting, or effective within the body.
Of, relating to, or located within the limits or surface; inner.