Eighteenth a. : Next in order after the seventeenth.

Eighteenth a. : Consisting of one of eighteen equal parts or divisions of a thing.

Eighteenth n. : The quotient of a unit divided by eighteen; one of eighteen equal parts or divisions.

Eighteenth n. : The eighth after the tenth.

Example:
What made republicanism a revolutionary ideology in the Eighteenth century?

Taxonomy
18th century, poet, philosopher, American statesman, English, English novelist, English philosopher, the French, French painter, French writer, Ottoman Empire, King, Japanese Shogun, English poet, China, Joseon Dynasty, revolutionary leader, scientist.

Defination from the web:
Eighteenth century) Jump to: navigation, search. Storming of the Bastille, July 14, 1789, an iconic event of the French Revolution ... The Long Eighteenth Century: British Political and Social History 1688-1832 (The Arnold History of Britain Series). A Hodder Arnold ...