A Martini is an
alcoholic drink made classically with gin and vermouth and served with a green
olive (when served with coctail onions the drink is called a "Gibson").
It is served chilled, often by ice in a specific shaker, then drained into the signature-style
glass.
The ratio of the two liquids is
5:1 (traditionally, 2 and one half ounces of gin to half an ounce of vermouth) usually, though by changing this, a person can make the drink "drier" (more gin) or "wetter" (less gin) depending on his tastes.
Martinis can also be made with other
alcoholic bases, the most common alternative to gin being vodka (often called a Vodka Martini, Vodkatini, or Kangaroo)
Other variations of the Martini centre around the inclusion of other alcohol or flavours in the drink.
Bill: Hey, want to go drinking tonight, you know, pick up a few foaties of Olde
English 800?
Steve: Nah, that stuff sucks balls. Let's get some Martinis down at the bar.
Bill: Sweet.