1. A unit of computer information expressed as a choice between two possibilities. a 0 or 1 in binary notation; short for binary digit. 2. Of a key, the projecting blade, cut in a manner that actuates the lock tumblers. 3. The removable cutting edge of a tool, as a drill bit. 4. Of a screw gun, the replaceable tip or portion that seats in the slotted screw head.