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.
In modern French, monsieur (plural messieurs) is used as a courtesy title of respect, an equivalent of English mister or sir. It can be abbreviated in M. (plural: MM.), Mssr. (plural: Mssrs.), and rarely M r (plural: M rs), but never Mr., which is only for Mister. See also. Dauphin; Fils de France; Madame Royale; Madame; Petit-Fils de France