プログレッシブ和英中辞典(第4版)の解説
よだん【余談】
a digression; an aside
余談はさておき当面の問題を検討しましょう
So much for digressions. Let's get back to the question at issue.
彼の話は余談に渡った
He digressed [wandered] from the main subject in his talk.
余談ですが，彼は料理の達人です
Incidentally, [I may add that] he is an excellent cook.
これは余談です
This is 「by the way [just an aside].