  The matter-enhanced oscillations of two neutrino flavors are studied using a
uniform semiclassical approximation. Unlike some analytic studies which have
focused on certain exactly-solvable densities, this method can be used for an
arbitrary monotonic density profile. The method is applicable to a wider range
of mixing parameters than previous approximate methods for arbitrary densities.
The approximation is excellent in the adiabatic regime and up to the extreme
nonadiabatic limit. In particular, the range of validity for this approximation
extends farther into the nonadiabatic regime than for the linear Landau-Zener
result. This method also allows calculation of the source- and
detector-dependent terms in the unaveraged survival probability, and analytic
results for these terms are given. These interference terms may be important in
studying neutrino mixing in the sun or in supernovae.
