This is a term that generally refers to an automotive dynamometer, but could also refer to an engine dynamometer. A dynamometer is a piece of equipment that is used to measure the power and torque output at the wheels of a car or crankshaft of the motor for an engine dynamometer. Most automotive tuners use an automotive dynamometer, which I will refer to from here on out. Dynos come in a variety of shapes and sizes, but the most popular dyno, is the Dynojet dynamometer. The Dynojet utilizes large drums of known inertia, mass, and circumference. Your car is straped down so that your drive wheels rotate the drums. Based on how fast and how quickly the drums are spun, a computer can calculate the power and torque that your car is creating. The dyno is one of the best tuning tools out there, and is the only way to quantify the actual gains of a particular performance modification. Keep in mind that a dyno measures power and torque at the wheels. This is what is truely important to you anyway, as this is the power and torque that drives your car, but note that between the crankshaft and wheels are the transmission and drive train. Each of these components introduces losses in output due to friction and inertia, so there is typically a 15% loss (for a manual transmission) in torque and power between the crank and the wheels. Automatic transmissions or AWD drivetrains incurr greater losses. The costs of running your car on a dyno are not cheap. A typically session of 2-4 runs may cost you $75-$100, or shops will commonly rent the dyno for $100-$200 per hour.