M10 Track Nut Rear, sold each
Thread is M10 x 1mm. This is largely a bike specific thread so you can't find these at Home Depot.
These nuts have beefy integrated serrated washers that help secure your wheel for singlespeed or fixed gear use.
This size is found on Novatec, Formula and many other good quality singlespeed rear hubs. BMX hubs and front hubs are different sizes. Price is for 1.