power chord

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

• noun A chord formation consisting of only two notes, usually the root and the fifth degrees of the scale, commonly used in rock music.

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

• noun idiomatic A chord or combination of notes used in rock music and typically selected to sound good at high volume and high levels of distortion. Power chords make extensive use of intervals such as open fourths and fifths.