The definition of an adjective is a part of speech that is used to describe a noun. Red, tall, better and fast are all examples of adjectives.
In modern parlance, a gentleman (from gentle + man, translating the Old French gentilz hom) is any man of good, courteous conduct. A gentleman may also simply be a polite term for any man, as in indications of gender-separated facilities, or as a sign of the speaker's own courtesy when addressing others. The modern female equivalent is lady.
The word gentleman, used in the wide sense with which birth and circumstances have nothing to do, is necessarily incapable of strict definition. For to behave like a gentleman may mean little or much, according to the person by whom the phrase is used; to spend money like a gentleman may even be no great praise; but to conduct a business like a gentleman implies a high standard.