BCD is the English term for bolt circle diameter. BCD is an abbreviation for Bolt Circle Diameter and describes a fixed measure of the distance between the holes in the chainring. The given value describes the diameter of the circle on which the bolt holes in the chainring are evenly spaced. The BCD of chainring and spider must match for mounting.
Chainrings and crank spiders with 130mm BCD for a classic gear ratio of 53/39 are frequently used in road cycling. Since a few years you can find more and more cranks and chainrings with 110mm BCD in combination with the more and more popular compact gear ratios 50/34 or 52/36. These chainrings are usually bolted 5 times.
In mountain biking and cyclocross, smaller bolt circle diameters in combination with 4-way bolting are common. Common sizes are 104, 96 or 64 mm. Occasionally there are also asymmetrically bolted or directly mounted chainrings without spider (Direct Mount).