A hole drilled into a wood surface prior to installing a screw in order to 1) prevent wood from splitting from large screws or when near the end or edge of the board, 2) make installing the screw easier in hard materials, such as oak or maple (especially with screws of softer material, like brass), and 3) to improve accuracy by providing a guide (or "pilot") path for the screw to follow.
Picture from www.diydoctor.org.uk
Picture from www.diydoctor.org.uk
