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.
A gentlemen's club, or formerly traditional gentlemen's club, is a members-only private club originally set up by and for British upper class men in the 18th century, and popularised by English upper-middle class men and women in the late 19th century and early 20th century.