It is just over a foot in diameter. I'm worried this fell of my 1999 Accord. I just moved in to my house and I've been finding trash here and there so it's also possibly a remnant from the old owners. I found it in the driveway.

 

Perhaps part of a brake dust shield, corroded and fell away.

good luck

 

Ditto. Looks like it might be the disc brake shield/guard. Number 18 in the diagram at this link: Part Detail

 

Thanks so much for helping me identify this!
How important is the brake dust shield, how quickly must it be replaced?
I'm wondering if I can do this on my own with a jack?

 

Looks like the wheel bearing assy must be removed to access. Shield protects against water splashing on hot rotor.

good luck

 

It doesn't have all the proper creases, so it might be the same part off someone else's car. Just check behind each wheel to see if it's missing. If yours are OK then don't worry.

 

Their not real important I cut all mine off when I changed my rotors their just in the way