An Etymological Dictionary of Astronomy and Astrophysics

The time within which the amplitude of an oscillation increases or decreases by a factor e (= 2.71828…).

Etymology (EN): From e the base of the natural, or Napierian, system of logarithms; folding, from -fold suffix meaning “of so many parts,” or denoting multiplication by the number indicated by the stem or word to which the suffix is attached (as in twofold; manifold), from O.E. -feald, related to Ger. -falt; Gk. altos, -plos, -plus; → time.

Etymology (PE): Zamân, → time; e, as above; tâyi noun of tâ multiplicative suffix, also “fold, plait, wrinkle; like, resembling.”