It's Glechoma hederacea.

This plant Glechoma hederacea,(creeping Charlie, ground ivy) is almost impossible to completely eradicate and you can scour the Internet for hundreds of ways to do it,but it will be no easy task.Dont let it get the best of you.On the bright side,it grows close to the ground,is an excellent ground cover and provides nectar for our disappearing bees,not to mention it smells nice ( minty).A lawn that is just a monoculture of only one grass species that is treated with synthetic fertilizers and herbisides is not a truly healthy lawn, and its soil structure will be unnatural and unhealthy.We borrowed the concept of the perfectly manicured lawns from the Europeans, but in the real world trying to maintain a perfect lawn is virtually impossible,not to mention very unhealthy. It's better to have a weed that stays low to the ground than have dandilions that keep sending up foot tall flower stems.