Well - it's a beautiful guilted General Service Eagle Button! Man what a looker! It appears to be made in the "muffin" style... These eagle buttons were the norm until 1902 when the U.S.Army adopted the current style - the U.S.Army Great Seal Button.
It's possible it saw action someplace but there is NO way to tell... for example, is your backyard a CW battlefield? A skirmish site? A training camp? A military camp of any kind? With no clues as to where you live it's kinda hard to tell... well, impossible to tell who lost it. Did a CW vet live in your house?
Also - the only way to date your button is to see the back mark... what's written on the back?
She's a beauty!