whammy bar

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

• noun A narrow bar attached to or near the bridge of an electric guitar that is depressed or raised to lower or raise pitch by changing the tension or length of the strings.

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

• noun music A lever at the bridge or tailpiece of an electric guitar, used to add vibrato to the sound by changing the tension of the strings.