What is noun in English?

A Noun is the name of a person, place or thing.

or

Name of anything is noun.

For example:

Sita is a nurse.

Ravi lives in Mumbai.

The honesty is super policy.

In above examples Sita and Ravi are name of persons, Mumbai is name of a place and Honesty is name of thing.

So, they are Nouns.
Other examples:

Pen, Table, Water, Gold, Silver, Kindness, India, Country, State etc.

Classification of Noun:

Noun is classified in 5 categories:

Proper Noun
Common Noun
Collective Noun
Material Noun
Abstract Noun