An orchidometer is a medical device for measuring the size of a testicle. The word is derived from orchis, the Greek word for "testicle," using the combining form orchido-.
Although the Greek makes it sound really fancy, the concept of the orchidometer is extremely simple. It is essentially a string of standard sized, wooden, testicle-shaped beads, ordered from smallest to largest, and often labled with numbers according to the standard scale for measuring testicle size.
To measure the size of a given testicle, compare it to beads in sequence, holding them side by side, until you find the bead which most closely matches the size of the testicle, and then record the result.
Orchidometers are often used in cases where a human or animal is on a regimen of anabolic steroids, in order to track testicle shrinkage.