18th century

The 18th century lasted from January 1, 1701 to December 31, 1800 in the Gregorian calendar. During the 18th century, the Enlightenment culminated in the French and American revolutions. Philosophy and science increased in prominence. Philosophers dreamed of a brighter age. This dream turned into a reality with the French Revolution, although it was later compromised by the excesses of the Reign of Terror of Maximilien Robespierre. At first, the monarchies of Europe embraced Enlightenment ideals, but with the French Revolution they feared losing their power and formed broad coalitions for the counter-revolution. The Ottoman Empire underwent a protracted decline, as it failed to keep up with the technological advances in Europe. The Tulip period symbolized a period of peace and reorientation towards European society, after victory against a burgeoning Russian Empire in the Pruth River Campaign. Throughout the century various reforms were introduced with limited success.

The 18th century also marked the end of the Polish–Lithuanian Commonwealth as an independent state. The once-powerful and vast kingdom, which was able to conquer Moscow and defeat the great Ottoman armies, collapsed under numerous invasions. Its semi-democratic government system was not robust enough to rival the neighboring monarchies of the Kingdom of Prussia, the Russian Empire and the Archduchy of Austria which divided the Commonwealth territories among them, changing the landscape of Central European politics for the next hundred years.

18 (number)

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

In speech, the numbers 18 and 80 are sometimes confused. When carefully enunciated, they differ in which syllable is stressed: 18 /eɪtˈtiːn/ vs 80 /ˈeɪti/. However, in dates such as 1864, or when contrasting numbers in the teens, such as 17, 18, 19, the stress shifts to the first syllable: 18 /ˈeɪttiːn/. In some dialects, such as General American, there is little confusion because the single t sound in 80 becomes a d-like flap [ɾ], whereas the double t sound in 18 does not.

In mathematics

Eighteen is a composite number, its proper divisors being 1, 2, 3, 6 and 9. Three of these divisors (3, 6 and 9) add up to 18, hence 18 is a semiperfect number. Eighteen is the first inverted square-prime of the form p·q².

It is a heptagonal number, and as the sum of the first three pentagonal numbers, it is a pentagonal pyramidal number. It is also the sum of the totient function for the first seven integers. It is the sixth lucas number.