The Collaborative International Dictionary of English v.0.48:

Blister \Blis"ter\, n. [OE.; akin to OD. bluyster, fr. the same
   root as blast, bladder, blow. See Blow to eject wind.]
   1. A vesicle of the skin, containing watery matter or serum,
      whether occasioned by a burn or other injury, or by a
      vesicatory; a collection of serous fluid causing a
      bladderlike elevation of the cuticle.
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            And painful blisters swelled my tender hands.
                                                  --Grainger.
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   2. Any elevation made by the separation of the film or skin,
      as on plants; or by the swelling of the substance at the
      surface, as on steel.
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   3. A vesicatory; a plaster of Spanish flies, or other matter,
      applied to raise a blister. --Dunglison.
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   Blister beetle, a beetle used to raise blisters, esp. the
      Lytta vesicatoria (or Cantharis vesicatoria), called
      Cantharis or Spanish fly by druggists. See
      Cantharis.

   Blister fly, a blister beetle.

   Blister plaster, a plaster designed to raise a blister; --
      usually made of Spanish flies.

   Blister steel, crude steel formed from wrought iron by
      cementation; -- so called because of its blistered
      surface. Called also blistered steel.

   Blood blister. See under Blood.
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The Collaborative International Dictionary of English v.0.48:

Cantharis \Can"tha*ris\ (k[a^]n"th[.a]*r[i^]s), n.; pl.
   Cantharides (k[a^]n*th[a^]r"[i^]*d[=e]z). [L., a kind of
   beetle, esp. the Spanish fly, Gr. kanqari`s.] (Zool.)
   A beetle (Lytta vesicatoria, syn. Cantharis vesicatoria),
   having an elongated cylindrical body of a brilliant green
   color, and a nauseous odor; the blister fly or blister
   beetle, of the apothecary; -- also called Spanish fly. Many
   other species of Lytta, used for the same purpose, take the
   same name. See Blister beetle, under Blister. The plural
   form in usually applied to the dried insects used in
   medicine.
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