It appears to be an uncommonly small version of what button collectors call a "brass 1-pieece flatbutton." That type often had a floral emlem on its front, so we relic-diggers call those a "flowerbutton." Yours seems to show a little of a manufacturer's mark ("backmark") encircling the stub of the broken thread-loop, so that dates your brass 1-piece flatbutton to the early-1800s. (Same thing without a backmark is 1700s-into-early-1800s.)
You might be wondering what such a tiny button was used for. One purpose we know of with certainty is, ladies' button-up hightop shoes.