十字 (shízì) literally means 'ten-character / cross-shaped' — because the character
十 looks like a plus sign. So
十字螺丝刀 = 'cross-tip screwdriver,' a perfectly logical name for a Phillips. The flat-head version is
一字螺丝刀 (yīzì, 'one-character / horizontal-line screwdriver') from the shape of
一.
