  The current short baseline reactor experiments, Daya Bay and RENO (Double
Chooz) have measured (or are capable of measuring) an effective $\Delta m^2$
associated with the atmospheric oscillation scale of 0.5 km/MeV in electron
anti-neutrino disappearance. In this paper, I compare and contrast the
different definitions of such an effective $\Delta m^2$ and argue that the
simple, L/E independent, definition given by $\Delta m^2_{ee} \equiv \cos^2
\theta_{12} \Delta m^2_{31}+ \sin^2 \theta_{12} \Delta m^2_{32}$, i.e. "the
$\nu_e$ weighted average of $\Delta m^2_{31}$ and $\Delta m^2_{32}$," is
superior to all other definitions and is useful for both short baseline
experiments mentioned above and for the future medium baseline experiments JUNO
and RENO 50.
