Here is an explanation, and picture.
A wheel's bolt-circle-diameter is the diameter of an imaginary circle drawn through the center of the wheel's mounting-bolt holes. Different vehicle makes and models have varying bolt patterns, so knowing how to measure bolt pattern is important if you're considering a move to aftermarket wheels.
If the pattern has an even number of mounting holes (four, six or eight lugs) simply measure from the center of one stud hole directly across the center of the wheel to the center of an opposite stud hole.
With a five-lug pattern, measure from the center of one stud hole to the center of the farthest stud hole, skipping the adjacent hole. The resulting measurement is slightly smaller than the actual bolt-circle diameter. The conversion table below displays the difference.