noun
monocular; telescope (single eyepiece)
A small telescope used with one eye, distinct from binoculars (双眼鏡).
バードウォッチングに単眼鏡を持っていく。
I take a monocular for birdwatching.
General term for telescope, often larger; 単眼鏡 is a compact monocular.
Compound of 単 (single) + 眼 (eye) + 鏡 (mirror/lens), literally 'single-eye mirror'.