Eighteenth/ˌeɪˈtiːnθ/ Definition 1 of 2Pro Tip 1/3Historical Context Used to refer to the 18th position in historical timelines, particularly centuries. The Industrial Revolution began in the eighteenth century. Pro Tip 2/3Milestone Celebration Commonly refers to significant birthdays or anniversaries, like turning 18. Her eighteenth birthday was celebrated with a grand party. Pro Tip 3/3Ranked Position Indicates precise order in a sequence, often in competitions or lists. He finished in the eighteenth place in the race.