heavy metal

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

• noun A metal with a specific gravity greater than about 5.0, especially one that is poisonous, such as lead or mercury.
• noun Music Very loud, brash rock music, often with shouted, violent lyrics.

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

• noun Any metal that has a specific gravity greater than about 5, especially one, such as lead, that is poisonous and may be a hazard in the environment.
• noun music A genre descended from rock music, characterized by massive sound, highly amplified distortion, and overall loudness, often with extended guitar solos, complex song structures, fast emphatic beats, and lyrics that involve violent or fantastic imagery.

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

• noun a metal of relatively high density (specific gravity greater than about 5) or of high relative atomic weight (especially one that is poisonous like mercury or lead)
• noun loud and harsh sounding rock music with a strong beat; lyrics usually involve violent or fantastic imagery