My understanding is it's any riding that replaces motorized transport. So, if you bike to work instead of drive, that counts. However, if you bike to the store instead of walk, that doesn't count. How you would have gone somewhere if you didn't take your bike is left to one's discretion. It's not scientific anyway. Some people track miles very precisely with GPSes and spreadsheets or mileage tracking sites. Others use SWAG or even WAG to arrive at their answers. Then there's the occasional "October surprise" entrant who makes no posts until the year is almost over and then somehow pops up near the top of the list.