"Pointer Down" refers to an event in user interface design that occurs when a user presses down on a touch screen or mouse button. This action signals the start of an interaction, such as selecting an item or dragging an object. It is a crucial part of touch and mouse input handling in applications and games. In programming, "Pointer Down" events are often used in frameworks like JavaScript or Unity to trigger specific functions. Developers can listen for this event to initiate actions, such as highlighting a button or starting a drag-and-drop operation, enhancing user experience and interactivity.