On BF, its usually your fastest sustainable speed +5-6 mph. Its usually preceded by the phrase, "I was cruising along..."

But seriously, it should be distance traveled / time you were on the bike. Some people leave out stopped time, some don't. Your computer may or may not stop the clock when you stop. Also keep in mind average times are fairly useless. You time on a crowded mup, or in the city with lights and traffic will be very different from your time on an empty country road in a group of 5-10 similarly fit riders.