Internal/external vs. intra/inter

[ekolis]

So the prefix "intra" means "within", while the prefix "inter" means "between" or "across"... but then we have "internal", which means "inside", as opposed to "external", which means "outside"! Why does "inter" mean completely opposite things in the words "internal" and, say, "internet" or "interstellar"?

[Makri]

How are "internal" and "between" complete opposites? "interstellar" doesn't mean "outside of stars", it means "between stars".

The connection you're looking for is probably this: if you take all stars together and mark the space that is delineated by them, something that is in that space is between the stars. I suppose that's why "in the mountain" doesn't mean "in mines".

[Mapar]

So the prefix "intra" means "within", while the prefix "inter" means "between" or "across"... but then we have "internal", which means "inside", as opposed to "external", which means "outside"! Why does "inter" mean completely opposite things in the words "internal" and, say, "internet" or "interstellar"?

The "inter" in "internal" has the same root. I've always interpreted it as meaning "contained between the boundaries of something".

Interstellar travel is travel between stars, the internet is a system of connections between computers, etc.

[Gigano]

I think it has something to do with the fact that the Latin adjective "internus" was used to describe domestic (i.e. internal) affairs. Internus itself is derrived from the preposition "inter" meaning 'between', but in the case of "internus" it would mean something along the lines of 'between cities, between districts". From internus the word "internalis" then followed. The suffix -alis when combined with internus creates internalis, meaning "pertaining to the inward part of something". The prefix inter- in internet and interstellar doesn't have this -alis suffix, adding to the two differences between internal (an adjective) and inter- (a prefix): a) different etymology (internus vs. inter), and b) missing the -alis suffix.