What is the Abstract noun for brighten?

Answer 1

The noun forms of the verb to brighten are brightener and the gerund, brightening.

• The noun 'brightener' is a concrete noun as a word for a substance added to products to increase the appearance of giving out or reflecting light; a word for a physical thing.
• The gerund 'brightening' is an abstract noun as a word for the process of creating an appearance of giving out or reflecting light; a word for a concept.

Answer 2

No matter what adjective describes it, the noun 'day' is an abstract noun.

All nouns for periods of time are abstract nouns. Time is a concept.