This is the 532nd most frequent Korean word.
눈에
"In the eye(s)" or "to the eye(s)."
Here, '눈에' (nune) means 'in the eyes of' or figuratively 'capturing the vision of'.
그녀의 아름다움이 모두의 눈에 띄었어요.
Her beauty caught everyone's attention.
In this context, '첫눈에' (cheonune) uses '눈에' (nune) as part of an idiomatic expression meaning 'at first sight'.
이 그림은 첫눈에 반할 만큼 매혹적입니다.
This painting is so captivating that it enchants at first sight.
Here, '눈에' (nune) literally means 'into the eye' in a spatial sense.