An aster of some type - there are many, many different species and hybrids. I'd look at the New England aster to start, Symphyotrichum novae-angliae.

Asters... Those look exactly like the wild ones we just ripped out yesterday as their flowering was finished.. They are very pretty but are vigorous seeders and spreaders. To maximize flowers, cut them back several times early in the season to make them shorter and bushier, and then let them be after early July to produce lots of buds - exactly like growing 'mums...!

You could also think about lifting and dividing - a fun job with asters, just as they are waking up in spring. Replanting will give the plants a fresh lease of life (and increase your supplies).

Rather looks like aster 'Purple Dome'. Do as Campanula advises to rejuvenate your plant.

'Purple Dome' aster