What Defines A PR???

We runners love numbers. Many of us keep meticulous training logs - in mine I track mileage, pace, average and maximum heart rate, which courses, which shoes (and accumulated mileage on my shoes) and weather - and I may have missed one or two pieces of data.

One thing that stands in the foreground of most runner's running logs is their list of Personal Records, or PRs, from races they have run.

Currently my list is:

Distance	Time	Pace	Event	Date
5 km	25:17	8:09	Kentlands 5K Race - New Course	8/30/2008
10 km	56:14	9:03	Pike's Peek	4/23/2006
Half Marathon	2:13:22.79	10:11	Run For The Shelter Half Marathon	4/19/2009
Marathon	4:59:06	11:25	Frederick Marathon	5/4/2008

However, during a few of my training runs, I have run faster than my PRs at the 10K and Half Marathon distances.

Do these become new PRs? Or should PRs only be for races or time trials dedicated to seeing how fast you can run a certain distance?

What's your thought?