i-adjective
Describes speech, writing, or movement that is uninterrupted and natural, without hesitation or awkward pauses.
彼の日本語はよどみなく、まるでネイティブのようだ。
His Japanese is fluent, just like a native speaker.
プレゼンはよどみない説明で、とても分かりやすかった。
The presentation had a smooth explanation and was very easy to understand.
流暢 is a noun/adjectival noun meaning fluent, often used for language ability. よどみない is an adjective that emphasizes the uninterrupted, smooth flow of speech or action.
スムーズ is a loanword meaning smooth, often used for processes or progress. よどみない is more literary and focuses on the absence of stagnation in expression.
From 淀み (stagnation) + ない (not), literally 'without stagnation'. The word is used to describe something that flows smoothly without pause.