A seamless bra is designed to be invisible under your clothes, with no seams that might show through. Seamless bras feature cups that are usually covered with a single piece of fabric that has no seams or stitching.
The cup is made out of a single piece of fabric and has no seams through the middle of the cup. (As opposed to a bra with the cups made of different pieces of fabric sewn together to form the cup shape).
Moulded bras may also have foam padding in the cups. Bras with foam padding are often called t-shirt bras.
A moulded bra cup holds its shape, even when not being worn. If you set it on a table, the cups will retain their dome shape.
Moulded bras get their name from the heat moulding process that makes the cups.
You'll find seamless bra designs in a variety of styles, including: T-shirt bras, sport bras, underwire, and wireless.