An Etymological Dictionary of Astronomy and Astrophysics

The dividing line between the illuminated and the un-illuminated part of the Moon’s or a planet’s disk.

Etymology (EN): From L. terminator, from terminare, from terminus→ term.

Etymology (PE): Šid-marz, literally “light boundary,” from šid “light, sunlight” (Mid.Pers. šêt “shining, radiant, bright;” Av. xšaēta- “shining, brilliant, splendid, excellent”)

• marz “boundary, limit” (Mid.Pers. marz “boundary;” Av. marəza- “border, district,” marəz- “to rub, wipe;” Mod.Pers. parmâs “contact, touching” (→ contact), mâl-, mâlidan “to rub;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
  Ger. Mark; E. mark, margin).