Source : Webster's Revised Unabridged Dictionary (1913)

Ambrotype \Am"bro*type\ (-t[imac]p), n. [Gr. 'a`mbrotos immortal
   + -type.] (Photog.)
   A picture taken on a plate of prepared glass, in which the
   lights are represented in silver, and the shades are produced
   by a dark background visible through the unsilvered portions
   of the glass.