An Etymological Dictionary of Astronomy and Astrophysics

Acoustics: Effect produced when sound is reflected or thrown back on meeting a solid obstacle.
Radio.:
A wave returned to the transmitter with sufficient magnitude and delay to be distinguished from the directly transmitted wave. In radar, the portion of the energy of the transmitted pulse reflected back to the receiver.

Etymology (EN): From L. echo, from Gk. echo, personified as a mountain nymph, from ekhe “sound.”

Etymology (PE): Pažvâk, literally “return sound,” from paž “back, against, opposite,” varaint pâd- (Mid.Pers. pât-, from O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Av. paiti, akin to Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti)

• vâk “sound,” Mid./Mod.Pers. vâng/bâng “sound, clamour;” Av. vacah- “word,” from vac- “to speak, say;” cf. Mod.Pers. vâžé “word,” âvâz “voice, sound, song” (Skt. vakti “speaks, says,” vacas- “word;”
  Gk. epos “word;” L. vox “voice;” PIE base *wek- “to speak”).