The 18th century lasted from 1701 through 1800 in the Gregorian calendar.
Historians sometimes specifically define the 18th century otherwise for the purposes of their work. For example the "short" 18th century may be defined as 1715-1789, denoting the period of time between the death of Louis XIV of France and the start of the French Revolution, while the "long" eighteenth century may run from the Glorious Revolution of 1689 to the battle of Waterloo in 1815 or even later.