eye ring
/ˈʌɪ rɪŋ/noun
1Chiefly Ornithology. A distinctively coloured ring surrounding the eye of an animal, especially a bird.
2A pin, bar, bolt, etc., with a ring or eye at one end.
3 Optics. A circular space within which the eye of the user of an optical instrument must be placed in order to obtain the full field of view.
Origin
Old English (in an earlier sense). From eye + ring.