  In this paper we generalize the concept of an effective $\Delta m^2_{ee}$ for
$\nu_e/\bar{\nu}_e$ disappearance experiments, which has been extensively used
by the short baseline reactor experiments, to include the effects of
propagation through matter for longer baseline $\nu_e/\bar{\nu}_e$
disappearance experiments. This generalization is a trivial, linear combination
of the neutrino mass squared eigenvalues in matter and thus is not a simple
extension of the usually vacuum expression, although, as it must, it reduces to
the correct expression in the vacuum limit. We also demonstrated that the
effective $\Delta m^2_{ee}$ in matter is very useful conceptually and
numerically for understanding the form of the neutrino mass squared eigenstates
in matter and hence for calculating the matter oscillation probabilities.
Finally we analytically estimate the precision of this two-flavor approach and
numerically verify that it is precise at the sub-percent level.
