An abstract noun (sometimes referred to as an intangible noun) is a noun which names anything which you can not perceive through your five physical senses, and is the opposite of a concrete noun. The highlighted words in the following sentences are all abstract nouns:

Buying the fire extinguisher was an afterthought.

Tillie is amused by people who are nostalgic about childhood.

Justice often seems to slip out of our grasp.

Some scientists believe that schizophrenia is transmitted genetically.