internal (adj.)

early 15c., "extending toward the interior," 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- (source also of 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 root *en "in."

Meaning "situated within" is from 1590s. Meaning "of or pertaining to the domestic affairs of a country (as in internal revenue) is from 1795; the notion is "pertaining to the subject itself; independent of others." Internal-combustion in reference to an engine in which fuel is burned inside it, is from 1884. Related: Internally.

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Definitions of internal from WordNet

internal (adj.)
happening or arising or located within some limits or especially surface;
internal party maneuvering
internal mechanism of a toy
internal organs
internal (adj.)
inside the country;
the nation's internal politics
Synonyms: home / interior / national
internal (adj.)
occurring within an institution or community;
Synonyms: intragroup
internal (adj.)
located inward; "Beethoven's manuscript looks like a bloody record of a tremendous inner battle"- Leonard Bernstein; "she thinks she has no soul, no interior life, but the truth is that she has no access to it"- David Denby; "an internal sense of rightousness"- A.R.Gurney,Jr.;
Synonyms: inner / interior
internal (adj.)
innermost or essential;
the internal contradictions of the theory
Synonyms: inner / intimate