A weed is a plant in the wrong place, it is called something else if it is in the right place. It is in your lawn and it is not grass, ergo it is a weed, though which one I am afraid I can't tell you. It looks like it would succumb to mowing, most things that don't make a circle of growth flush to the ground do. You could take a bit out and grow it on elsewhere in case it is not a weed in another situation, flowers are the easiest route to identifying things, but I doubt it is really worth it.