Micrometer, screw gauge instrument for determining the size of components to within 0.025 mm or less. (0.001 = a micron. Which is one millionth of a metre, or equivalently one thousandth of a millimetre).

The measurement is made between a fixed abutment and the end of a spindle which has an accurately machined thread and screws into the body of the instrument. Each complete turn of the spindle causes it to advance through a distance equal to the pitch of the thread, and a sleeve attached to the spindle has a graduated circumference.