The Flowy Side Slit Racerback Tank from Bella.  Featherweight and crazy comfy 65/35 poly/cotton blend.

"The newest addition to our top-selling flowy collection, this tank has side slits and a high-low, rounded hem for a super-chic finish. Relaxed and ready-for-anything, it features a scoop neck, and side seams. Sideseamed. Relaxed, draped fit. Side slits with high-low rounded hem."

More style and sizing info here.