skull and crossbones

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

• noun A representation of a human skull above two long crossed bones, used traditionally as a symbol of death or as a warning of the danger of death.

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

• noun An outline of a human skull and two crossed bones; a symbol of death traditionally used on the Jolly Roger, but now used as a warning on poisons.

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

• noun emblem warning of danger or death