noun Usually cross hairs.
Examples from the Web for cross hair
Then the screw was turned till the cross-hair bisected the deflected image of the slit.
After the cross-hair indicators and the level have been attached to the camera, adjustments are necessary.The Boy Mechanic, Book 2|Various
The "radius of measurement" is the distance between the front face of the revolving mirror and the cross-hair of the micrometer.