Fifty/ˈfɪfti/ Definition 1 of 2Pro Tip 1/3Age Milestone Often used to mark significant birthdays, especially the 50th year. My parents threw a huge party when they both turned fifty. Pro Tip 2/3Percentage Usage Used to describe half of something, often in sales or statistics. Everything in the store is fifty percent off today. Pro Tip 3/3Common Expression 'Fifty-fifty' means something is divided equally between two parties. We went in on the gift fifty-fifty, so we both paid the same amount.