Thirteenth/ˌθɜːrˈtiːnθ/ Definition 1 of 3Pro Tip 1/3Superstition Link Some people consider the number thirteen unlucky, affecting how it's viewed in different cultures. There's no thirteenth floor in many buildings. Pro Tip 2/3Not Just Dates Besides dates, 'thirteenth' can refer to order in lists, positions, or events. She stood in the thirteenth spot in line. Pro Tip 3/3Birthday Milestone 'Thirteenth' can mark special occasions, like transitions from childhood to teenage years. Sam's thirteenth birthday was a big event.