A "weed" is any plant you do not want growing where you do not want it to grow, and that can cover dandelions or most any wild flower. There are places in the lawn that have Forget Me Nots growing and many people would consider these "weeds" and try hard to eliminate them while they seem to pretty prolifically self seed. Then there are people that try to grow grasses in their lawns but yank it out of their flower beds with a vengance. I also have self seeded Rosa rugosa growing many places and I keep cutting these down because they are growing where I don't want them.

I think it might be the first year foliage of Hackelia virginiana, Stickseed, a native woodland plant. I think you will consider it a weed, especially if you have a dog! Its second year (minuscule) flowers turn into the stickiest burrs you can encounter. When I first found these, I would think it might be a baby Purple Cone Flower, and have to go compare the stem and foliage side by side to make sure it wasn't.

If it's not interfering with anything, you can always leave it for next year to be sure. If I'm right, it will bolt and flower (for what it's worth), and then just make sure you yank it! I always found them easy to pull.