Birthday/ˈbɜːrθdeɪ/ Definition 1 of 3Pro Tip 1/3Cultural Significance Celebrating birthdays is common in many cultures, often involving parties and gifts. In the USA, sweet sixteen is a significant birthday celebration. Pro Tip 2/3Age Milestones Certain ages are especially celebrated as milestones, like 18th and 21st birthdays. My 18th birthday was a big event with family and friends. Pro Tip 3/3Birthday Expressions Phrases like 'Happy Birthday' or 'Many happy returns' are common birthday greetings. We all sang 'Happy Birthday' to him when he cut the cake.