Originally: a small telescope for use outdoors (now historical). Later: (in plural and less commonly singular) a hand-held optical instrument for viewing distant objects outdoors and having a lens for each eye, so as to give a stereoscopic view; a pair of binoculars.
Binocular opera glasses using mirrors (Galilean optics) were devised in the early 19th century. The use of prisms was first proposed 1854 and they were developed commercially by the firm of Zeiss in the 1890s.
field glass/ˈfiːld ɡlɑːs/ /ˈfiːld ɡlas/
Late 18th century. From field + glass.