<p><strong>Verb</strong></p><ul><li>(Used with object) To enlarge the upper part of (a cavity), especially by chamfering, to receive the cone-shaped head of a screw, bolt, etc.</li><li>To cause (the head of a screw, bolt, etc.) to sink into a prepared depression so as to be flush with or below the surface.</li></ul><p><strong>Noun</strong></p><ul><li>A tool for countersinking a hole.</li><li>A countersunk hole.</li></ul>