A shirt that has no shoulder straps, attached sleeves, halterneck or other shoulder- or collarbone-related attire holding up the upper portion of the shirt, instead relying on the tightness of the shirt's neckline to keep it in place. The shoulders and upper chest area should be bare.
For the shirt to qualify as a strapless shirt, the shirt must not have attached sleeves or held up by straps. If sleeves are attached, then the shirt is an off-shoulder shirt. If it is held up by straps, then it is a camisole.