18 (eighteen) is the natural number following 17 and preceding 19.

In mathematics

Eighteen is a composite number, its proper divisors being 1, 2, 3, 6 and 9. It is a heptagonal number, and as the sum of the first three pentagonal numbers, it is a pentagonal pyramidal number. It is a Harshad number.

In science

• The atomic number of argon

In astronomy,

Messier object M18, a magnitude 8.0 open cluster in the constellation Sagittarius

The New General Catalogue object (http://www.ngcic.org/) NGC 18, a double star in the constellation Pegasus

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -2416 June 2 and ended on -1118 July 21. The duration of Saros series 18 was 1298.1 years, and it contained 73 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -2078 April 2 and ended on -780 May 21. The duration of Saros series 18 was 1298.1 years, and it contained 73 lunar eclipses.

In human affairs: Age 18

In many parts of the world, 18 is the age minimum for being allowed a driver's license and/or for purchasing alcohol or tobacco. It is also the voting age in many countries. During the Vietnam War, Americans could be drafted at an age as low at 18, but were not allowed to vote in most states until they were 21. A movement spread to lower the voting age, and in 1971, the Twenty-sixth Amendment extended a guarantee of suffrage to anyone 18 years or older in all states. The voting age was lowered from 21 to 18 in the U.K. and many other nations around that time.

Eighteen is also the age in most countries at which a person may appear in a pornographic video, if pornography is legal at all, or have sex with a person in a position of authority over them (a teacher, athletic coach, or employer, for instance, or in general anyone else over the age of 18).

In Judaism

In Hebrew Numerology, the word for life (In Hebrew language "Chai" - חי), adds up to 18. As a result, the number 18 is believed to be lucky and donations, and monetary gifts are normally given in denomonations of 18.

Other fields

Eighteen is also used in neo-Nazi circles as a code word for Adolf Hitler. The number comes from the position of the letters in the alphabet: A = 1, H = 8. See also 88.

Eighteen is also:

• 18, the title of an album by Moby.
• The number of holes in a game of golf.
• A slang term used in Soccer for the Penalty Area.
• The number of wheels on the most common type of tractor-trailer truck, which are hence often called 18-wheelers.
• Any of various historical years: 18 A.D., 18 B.C., or 1918.
• The customary percentage to tip a waitress in a restaurant by groups of four or more
• The number of the French department Cher and the Turkish province Bitlis