  Neutrino oscillation is discussed with emphases placed more on its conceptual
aspects. After reviewing the two conventional formulations, referred to here as
the same-energy prescription and the same-momentum prescription, wave packet
treatments are developed for each of these two prescriptions. Both wave packets
localized in space and those in time are considered, and, by invoking
relativistic kinematics as well, the necessary conditions for oscillation to
occur are derived, which appear to have a form more well-defined and
quantitative than what have been noted before. Some phenomenological
implications suggested by the wave packet treatments are briefly mentioned.
Finally, as a possible third prescription, the same-velocity prescription is
given.
