Wiktionary
crop top
n. A short T-shirt or blouse that is cut off above the abdomen.
Wikipedia
Crop top
A crop top (also cropped top, belly shirt, half shirt, midriff shirt, midriff top, tummy top, short shirt, and cutoff shirt) is a top, the lower part of which is high enough to expose the waist, navel or some of the midriff. The cropping of a top in this manner is generally limited to female garments. Bikinis and sports bras are generally not regarded as crop tops.