Nails, jogger's: Very common, small, semicircular white spots on the toenails. These spots result from injury to the base (matrix) of the nail, a structure under the visible nail where the cells that make up the visible nail are produced. The injury responsible for these white spots on the nails can be due to athletic activity or poorly fitting shoes; jogging in poorly fitting shoes causes the condition so often as to coin the term jogger's nails. These spots are not a cause for concern. They eventually grow out with the nail.