The word describes the effect: the outer line of the product stands unbroken against an empty ground. Nothing overlaps it, nothing props it up visibly, and the background carries no scene. Silhouette, packshot and cut-out overlap in use; silhouette puts the weight on the readable outline.
A clear silhouette is what makes a product recognisable at thumbnail size and comparable in a row of search results. It depends on contrast between product and ground, which is why white, cream and transparent products are the hard cases, and on an arrangement in which straps, handles and cables do not cross the body.
Marketplaces that ask for a silhouette want it as the first image or beside images that show the product in a setting. Photographing the product on a contrasting surface, with its whole edge visible, gives background removal a clean line to follow.