An abstract noun is a noun that you cannot sense, it is the name we give to an emotion, ideal or idea. They have no physical existence, you can't see, hear, touch, smell or taste them, but you can experience them. The opposite of an abstract noun is a concrete noun.
Justice, an idea, bravery, and happiness are all abstract nouns.
Here is an a-z list of some of the more common abstract nouns:-
See also Concrete Nouns
Thanks to DP for the correction.