Meaning of decay time

decay' time"

Pronunciation: [key]
— Physics. Physics.
  1. the time required for a collection of atoms of a particular radionuclide to decay to a fraction of the initial number equal to 1/e. Cf. e (symbol) (def. 1). Also called
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.