A word expressing a thing, place, person, quality or state of condition, position or attitude is called a Noun.
In other words a noun is the name of a person, place,thing or a quality. It is generally formed of one word like book, crow or honesty etc.
But sometimes it is formed of more than one words like Bookworm or Armchair etc. Book and worm separately are nouns and have their meanings but these two combined also form a noun which has a different meaning.A compound noun is of three words also like Father - in - law.