The Short Answer
A prep sink is a secondary kitchen sink — typically a small single-basin undermount sink — located in an island, peninsula, or prep zone away from the main sink. It allows one person to prep food (washing vegetables, filling pots) while another handles cleanup at the main sink. Prep sinks are typically 15-18 inches wide, much smaller than the main kitchen sink. Adding a prep sink to an island requires plumbing rough-in (supply lines and a drain) routed through the island cabinetry and cabinet floor to the main drain stack.