I feel compelled to add a bit of information to m_turner's fine and informative writeup above. The martini may be the only drink in the world which has received the solemn respect of standardization. The American Standards Association (predecessor to the American National Standards Institute), whose initials live on in photographic film ratings, ceased operations in 1966 before morphing into the aforementioned ANSI. The last published standard from that body was, in point of fact, American Standard K100.0 1966: Safety Codes and Requirements of American Standard Dry Martinis. This opus was produced by Sectional Committee K100 on Liquids Management, and was approved as a standard in August of 1966.

Thanks to George B. Kaufmann, writing in the journal Chemical Educator in 2001¹, we have the following excerpt, which defines some of the oft-debated characteristics of the drink:

Dry Martini: A cocktail made with English or American dry gin of at least 86 proof and dry vermouth, preferably French in origin, in accordance with requirements of this American Standard.

Gibson: An unpardonable form of perversion. See Onion Soup.

Lemonade: A term applied to drinks which have been subjected to the peel of a lemon. There is no place for the rind of any citrus fruit, or its oils, in an American Standard dry martini.

Onion Soup: the unholy abomination produced by the introduction of one or more pickled onions into a dry martini cocktail.

Rocks: the solid state of H₂O on which an American Standard dry martini is never served.

Vodka: A distilled alcoholic beverage made originally from potatoes, but now encountered in grain alcohol versions. It may be clean, palatable, and nonlethal, and when encountered in this form, is a fitting accompaniment for fresh caviar. It is never employed in a dry martini.

2.1 Basic Nomenclature: The American Standard dry martini shall come in the following three sizes:

• Regular–not less than 3.5 oz.
• Large–not less than 5 oz.
• Double–not less than 7 oz.

3.1 General: Only the following three ingredients shall be used in the preparation of the American Standard dry martini.

• Gin
• Dry vermouth
• Olives

3.3.1 Use of Vermouth: The employment of vermouth in an American Standard dry martini shall not be mandatory, provided no other ingredient is employed as a substitute.

3.4 Olives: While the use of olives is not encouraged, nothing in this specification shall be construed to mean that the inclusion of an olive will not be accepted, provided it conforms to Table 1 and subparagraphs 3.4.1 and 3.4.2.

I have been fruitlessly searching for an actual hardcopy of this standard for years and years. I'm told by a couple of engineers who have seen it that the copies they have seen have been enshrined in libraries and trophy cases, so the likelihood of my finding one is low - but we do what we must, because we can.

Notes:

1 - Kauffman, George B. "The Dry Martini: Chemistry, History, and Assorted Lore." Chemical Educator 2001.6 (2001) 295-305.