http://foldoc.org/argument even includes the distinction between "formal argument" and "actual argument".

For the situations you seem to be describing, I tend to use an adjective rather than a noun so that the noun can clarify the type of argument. "The passed-in string will be made all upper case" or "The specified subdirectory will be created" or "given".

Interestingly, Foldoc defines parameter as a synonym only for "formal argument" even though I use it and see it used as "actual argument" quite often.

Does that lead to transitus, transita, transeuntes, transeuntes, et cetera? And does that lead to a lakh of list wars about pluralization and doubts of arguments? I think your title mostly answers itself: is there a correct equivalent for 1?

(Not trying to be super snarky, just having fun! I think it's a good question to ask: "argument" is probably the best choice as it's in wide use and in JS, at least, there is even an arguments object in functions.)

I think we should form a committe that will be in charge of issues like pluralization and propagating this terminology to the masses. My 2 cents for the plural form of transitum is transita but I lack a broad background in Latin.

Hmm, list wars are inevitable; see Clausewitz. But because transitum is a neuter singular, the only possible plural is transita.

I think that the most accepted word is "argument" with "parameter" being less common but still used. I do really like "transitum". Maybe we should try to make that stick.