Bolt Circle Diameter. If you scribed a circle around your cranks, with the center at the middle of the bb axle and the circumference passing thru the center of all the bolt holes of the chainring, the diameter of that circle would be the bcd.

Standard road (and track) bcds:

Shimano road: 130mm

Campag road: 135mm

Standard track (also Campag Nuovo Record road, back in the 70's): 144mm

Old style Campag (going way back): 151mm (figure that one out!)

Compact cranks: 110mm

Luis