quartz crystal

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

• noun A small crystal of quartz accurately cut along certain axes so that it can be vibrated at a particular frequency, used for its piezoelectric properties to produce an electric signal of constant known frequency.
• noun Any of several crystalline forms of quartz.

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

• noun a thin plate or small rod of quartz cut along certain lines and ground so that it can produce an electric signal at a constant frequency; used in crystal oscillators