An ambrotype is a positive photograph on glass made by a variant of the wet-collodion process. James Ambrose Cutting patented the ambrotype process in 1854. It largely replaced the slower, more expensive daguerrotype. In the 1860s it was itself superseded by the tintype.  Web resources here and here.

Unidentified African-American soldier in Union uniform with wife and two daughters. Ambrotype. 1863-65. Library of Congress, Washington, DC.