A mildly pleasurable and euphoric state caused by running or other activity, whereby natural opiates are supposedly released into the bloodstream and act as painkillers. It has been described as

...you get so into your run that you feel really great, lose total track of time, and are very aware of the sights around you as you progress through your run. You feel very free and at ease with yourself

Not all scientists believe in the runner's high, which typically occurs after about 5 miles.