na-adjective
Literary expression emphasizing extreme brevity, like dew that vanishes quickly. Often used in set phrases such as 露の間も忘れない (not forget even for a moment).
露の間も彼のことを忘れたことはない。
I have never forgotten him even for a moment.
その美しさは露の間の夢のように消えた。
That beauty vanished like a fleeting dream.
一瞬 is a common, neutral word for 'an instant'. 露の間 is more literary and poetic, emphasizing transience.
束の間 also means 'a brief moment', but is more common in everyday speech. 露の間 is rarer and more literary.
Compound of 露 (つゆ, 'dew') + の (possessive particle) + 間 (ま, 'interval, time'). The image is of a time as brief as the morning dew, which evaporates quickly.