The Collaborative International Dictionary of English v.0.48:

Breastplate \Breast"plate`\, n.
   1. A plate of metal covering the breast as defensive armor.
      [1913 Webster]

            Before his old rusty breastplate could be scoured,
            and his cracked headpiece mended.     --Swift.
      [1913 Webster]

   2. A piece against which the workman presses his breast in
      operating a breast drill, or other similar tool.
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   3. A strap that runs across a horse's breast. --Ash.
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   4. (Jewish Antiq.) A part of the vestment of the high priest,
      worn upon the front of the ephod. It was a double piece of
      richly embroidered stuff, a span square, set with twelve
      precious stones, on which were engraved the names of the
      twelve tribes of Israel. See Ephod.
      [1913 Webster] Breastplow

WordNet (r) 3.0 (2006):

breastplate
    n 1: armor plate that protects the chest; the front part of a
         cuirass [syn: breastplate, aegis, egis]

Easton's 1897 Bible Dictionary:

Breastplate
   (1.) That piece of ancient armour that protected the breast.
   This word is used figuratively in Eph. 6:14 and Isa. 59:17. (See ARMOUR.)

     (2.) An ornament covering the breast of the high priest, first
   mentioned in Ex. 25:7. It was made of embroidered cloth, set
   with four rows of precious stones, three in each row. On each
   stone was engraved the name of one of the twelve tribes (Ex.
   28:15-29; 39:8-21). It was in size about ten inches square. The
   two upper corners were fastened to the ephod by blue ribbons. It
   was not to be "loosed from the ephod" (Ex. 28:28). The lower
   corners were fastened to the girdle of the priest. As it
   reminded the priest of his representative character, it was
   called the memorial (28:29). It was also called the breastplate
   of judgment (28:15). (See PRIEST.)