Bolt Center Diameter. Literally, if you were to draw a circle through the center of the chainring bolts, the diameter of that circle is the measure referred to. Usually specific to a manufacturer. Easier to research what goes with your crank than to try and measure yourself.
'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
Four 4-bolt chainrings the BCD is easy to measure, just measure the distance across two opposing bolts. For 5-bolt cranks it's not so straight forward so the usual method is to measure the distance between two adjacent bolts and look up a conversion chart such as Sheldon Brown's.