The phrase "km diameter" means the distance from one side of a circle or sphere to the other side, measured in kilometers. It tells us the size or width of the object. Full definition
Measuring some 50 to 150 kilometers across, the new moons are much smaller than Charon, the 1192 - km diameter satellite of Pluto discovered in 1978. (sciencemag.org)
Data from Dawn's visible and infrared mapping spectrometer show an unusually high concentration of organic matter close to the 50 - km diameter Ernutet crater in the northern hemisphere of Ceres. (sciencedaily.com)
Some 66 million years ago, a 10 - km diameter chunk of rock hit the Yukatan peninsula near the site of the small town of Chicxulub with the force of 100 teratons of TNT. (sciencedaily.com)