The phrase "average diameter" refers to calculating the size of something by measuring its width in different places and then finding the average of those measurements. It gives you an overall idea of how big something is. Full definition
The surface of Ceres, whose average diameter is 584 miles (940 kilometers), is generally dark — similar in brightness to fresh asphalt — study authors wrote. (astrobio.net)
The whey protein concentrate undergoes a unique process, resulting in uniform protein particles with an average diameter of one micron. (cpkelco.com)
Phobos has an oblong shape with an average diameter of about 14 miles (22 kilometers). (sciencedaily.com)