A personal record or personal best (frequently abbreviated to PR or PB) is an individual's best performance in a given sporting discipline. It is most commonly found in athletic sports, such as track and field, other forms of running, swimming and weightlifting.
The term "PR" came from the world of running, referring to a person's best time in a race of a specific distance. So, if someone runs their first 5K race in 28:45, that's their PR for the 5K distance. If they run faster than 28:45 in a subsequent 5K race, then they have a new PR for that distance.
Over time, "PR" came to mean any new record in a sport that could be performed by a single person. Crossfit and powerlifting have popularized the term because a person's PR is used to measure their current level for a given exercise.