An Etymological Dictionary of Astronomy and Astrophysics

Continual rise and fall.

Etymology (EN): Verbal noun of → fluctuate.

Etymology (PE): Oftâxiz “fall and rise,” from oft “fall”
stem of oftâdan “to fall” (Mid.Pers. opastan “to fall,” patet “falls;” Av. pat- " to fly, fall, rush," patarəta- “winged;” cf. Skt. patati “he flies, falls,” pátra- “wing, feather, leaf;” Gk. piptein “to fall,” pterux “wing;” L. penna “feather, wing;” O.E. feðer “feather;” PIE base *pet- “to fly, rush”) + -â- interfix

• xiz present stem of xâstan, xizidan “to rise, get up” (Mid.Pers. xyz- “to stand up, rise;” Proto-Iranian *xiz- “to rise, ascend; increase”).