Specifically, it is a US Army collar-disc insignia. Yours looks like the 1910-to-1924 version, but I can't be 100%-certain, due to it being uncleaned. If its face is slightly domed, it is from 1926-to-1937. To date it precisiely, you can search for an exact match to it here: Dating Metallic Insignia: Collar Disks