Is It A Flower or Weed?

Here is a photo of one of the first blooms in my yard. When I moved a few years ago, I planted all perennials in my garden in the front of my house. This particular photo is a flower I never planted. It pops up every year and it is really pretty. I am wondering if it is really a flower or just some type of pretty weed. The blooms are really small (about the size of a quarter), and I have never seen any type of flower like this. Inquiring minds want to know...is it a flower or a weed?