center punch

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

• noun A tool with a sharp point used in metalworking to create an indentation that marks the center of a hole to be drilled and steadies the drill bit when drilling.

from The Century Dictionary.

• noun A tool consisting of a small piece of steel with a hardened point at one end, used for making an indentation, such as to mark the center of a hole to be drilled or a circle to be struck, or as a center of revolution in a lathe. Also called dot-punch and prick-punch.

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

• A punch for making indentations or dots in a piece of work, as for suspension between lathe centers, etc.
• A punch for punching holes in sheet metal, having a small conical center to insure correct locating.

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

• verb to create a small hole or indentation in a workpiece as a guide in preparation for drilling
• noun countable a device used to create a small indentation in preparation for drilling a hole
• noun countable the small indentation created by such a device

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb make a small hole in something as a guide for a drill
• noun a tool with a conical point that is used to make indentations in metal (especially to mark points for drilling)