The noun 'curiosity' is an abstract noun as a word for a strong desire to know or learn something, a word for an emotion; a word for an odd or interesting fact, a word for a concept.
Example sentence: It was curiosity that led me to this website.
The noun 'curiosity' is a concrete noun as a word for a strange or unusual object; a word for a physical thing.
Example sentence: It looks like a curiosity from an ancient culture.