Source : Webster's Revised Unabridged Dictionary (1913)

Skull \Skull\, n. [OE. skulle, sculle, scolle; akin to Scot.
   skull, skoll, a bowl, Sw. skalle skull, skal a shell, and E.
   scale; cf. G. hirnschale, Dan. hierneskal. Cf. {Scale} of a
   balance.]
   1. (Anat.) The skeleton of the head of a vertebrate animal,
      including the brain case, or cranium, and the bones and
      cartilages of the face and mouth. See Illusts. of
      {Carnivora}, of {Facial angles} under {Facial}, and of
      {Skeleton}, in Appendix.

   Note: In many fishes the skull is almost wholly cartilaginous
         but in the higher vertebrates it is more or less
         completely ossified, several bones are developed in the
         face, and the cranium is made up, wholly or partially,
         of bony plates arranged in three segments, the frontal,
         parietal, and occipital, and usually closely united in
         the adult.

   2. The head or brain; the seat of intelligence; mind.

            Skulls that can not teach, and will not learn.
                                                  --Cowper.

   3. A covering for the head; a skullcap. [Obs. & R.]

            Let me put on my skull first.         --Beau. & Fl.

   4. A sort of oar. See {Scull}.

   {Skull and crossbones}, a symbol of death. See {Crossbones}.

Source : WordNet®

skull and crossbones
     n : emblem warning of danger or death