This is the 4595th most frequent Japanese word.
快
"快" means "pleasant," "comfortable," or "joyful."
The word '快適' means 'comfortable,' emphasizing a pleasant state that supports relaxation.
この部屋は快適で、くつろぎやすいです。
This room is comfortable and easy to relax in.
The word '快い' means 'pleasant,' describing an enjoyable or agreeable feeling induced by someone or something.
彼の笑顔は見る者に快い印象を与えます。
His smile gives a pleasant impression to the observer.
Again, '快適' indicates comfort and convenience, applied here to usability and interface design.