but then; although; though~
A conjunction meaning 'but then' or 'although', used to introduce a nuance of concession or partial exception to what was just said.
This word is used to qualify or soften a previous statement, similar to 'though' or 'but then' in English. It often introduces a small exception or a point that limits what was just said. For example, you might say something positive about a plan, then add もっとも to note a drawback. It is more common in written or formal spoken Japanese than in casual conversation, where you might use ただ or でも instead.
もっとも can also be used as an adverb meaning 'most', but in this pattern it functions as a conjunction. The nuance is that the speaker is making a concession or adding a caveat, not contradicting outright.