A lifetime is the entire span of time that someone or something exists, from beginning to end. Your lifetime started when you were born and will last as long as you live. A butterfly's lifetime might be just a few weeks, while a giant tortoise's lifetime can stretch beyond 150 years.

When people talk about a once-in-a-lifetime opportunity, they mean something so rare and special that you might only get one chance at it in your entire life. A trip to see the Olympic Games in person, meeting your hero, or witnessing a total solar eclipse from the path of totality might be once-in-a-lifetime experiences.

The word also describes how long objects last. A high-quality tool might have a lifetime guarantee, meaning the company promises it will work for as long as you own it. A lifetime supply of something means enough to last you your whole life, though advertisers sometimes use this phrase loosely.

People also use lifetime to describe major accomplishments: a scientist might call a discovery their lifetime achievement, meaning the crowning work of their entire career.