The martini is a cocktail made with gin (sometimes vodka #thanksianflemming) and vermouth. A cocktail by definition must have at least 3 ingredients, so the garnish is integral to the flavour profile of the martini.
Sometimes the term "martini" (or the suffix -tini) is used to refer to other cocktails, whose only commonality with the drink is the cocktail glass in which they are served. #appletini #espressomartini