planometer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A flat metal plate for gauging the accuracy of a plane surface in precision metalworking.

from The Century Dictionary.

• noun A plane surface used in machine-making as a gage for plane surfaces; a surface-plate.

from the GNU version of the Collaborative International Dictionary of English.

• noun An instrument for gauging or testing a plane surface. See Surface gauge, under surface.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A flat, metal plate used for estimating the flatness of a machined part; a surface plate