An Etymological Dictionary of Astronomy and Astrophysics

The distance between the eyepiece of a telescope and the location of the exit pupil.
This is where the observer’s eye should be positioned to see the entire field of view of the eyepiece. Also termed eye distance.

Etymology (EN): → eye; relief, from M.E. relef, from O.Fr. relief “assistance,” from relever “to raise,” from L. relevare “to raise, alleviate,” from re- intensive prefix, + levare “to lift up, lighten.”

Etymology (PE): Cašm nehâd “eye position,” from cašm, → eye, + nehâd “position, placing, posture,” contracted form of nehâdan “to place, put;” Mid.Pers. nihâtan;
Av. ni- “down; into,” → ni-,

• dā- “to put; to establish; to give,” dadāiti “he gives;” cf. Skt. dadāti “he gives;” Gk. didomi “I give;” L. do “I give;” PIE base *do- “to give.”