A T-shirt bra is a bra you can wear under fitted T-shirts or any clothes when you'd rather have a smooth look. T-shirt bras are usually simple in terms of style without lace or embroidery on the cups, as this means they are seamless under your clothes.
The biggest tell tale sign of a t-shirt bra: smooth, seamless cups. Keep in mind that some t-shirt worthy bras may not be molded or padded. These seam-free, stretchy or woven cup bras are great for wearing under t-shirts, but are typically not included in the t-shirt bra category and they are available in all cup sizes.