Wheel size. Eg: 622

Unfortunately, marketing people often use terms like 700c, 26 inch, or 29er to talk about wheel size. These common terms were meant to specify the diameter of the wheel including the tire. The proper way to specify rim size is without the tire, using the inner diameter where the bead of the tire sits. This is called "Bead Set Diameter" or BSD. The "bead" is the metal hoop part of the tire that presses against the rim when the tire is inflated. The sizes commonly referred to as 700C or 29 inch are both 622mm BSD.

 Nominal      BSD   Common Use
--------------------------------
  26 inch     559 mm    mountain bikes up to a couple of years ago
  700C        622 mm    all road bikes, hybrids, and 29er mtn bikes
  27.5 inch   584 mm    some mountain bikes, wierd size