Janka Hardness Test
Janka hardness test measure the hardness or density of wood. Testing is done on samples, not on actual flooring. It measures the force required to embed a 0.44" steel ball into the wood until half of the steel ball is into the wood. It was developed as a variation of Brinell test (measurement method of hardness of metals). Factors affect how flooring performs with type of core (for engineered flooring), grain direction and thickness of surface.
Results of measurements are stated in various ways for different countries. In United States, the measurement is in pounds-force (lbf). Other countries may state the test result in different units: kilogram-force (kgf), newtons (N), kilonewtons (KN), or simply treated as units (ex. "660 Janka").
For more information on Janka test, please follow this outside link to USDA's Forest Service, Research and Development: Forest Product Laboratory.