The disector is a 3-D geometric probe for counting numbers of objects (cells) with a probability that is unaffected by the size, shape, or orientation of the objects. In combination with Gundersen's unbiased counting rules and unbiased counting frames to avoid edge effects, the disector principle by D.C. Sterio in 1984 was the first method to allow unbiased estimates of total numbers of cells in a known volume, without assumptions, models, or correction factors.

Practical applications of the disector principle include counting objects with two physical planes (physical disector), two optical planes (optical disector), and optical planes in conjunction with the fractionator sampling scheme (optical fractionator).