  We show that the $3\times 3$ lepton flavor mixing matrix $V$ can be expanded
in powers of a Wolfenstein-like parameter $\Lambda \equiv |V_{\mu 3}| \sim
1/\sqrt{2}$, which measures the strength of flavor conversion in atmospheric
neutrino oscillations. The term of ${\cal O}(\Lambda^2)$ is associated with the
large mixing angle in solar neutrino oscillations. The Dirac phase of CP
violation enters at or below ${\cal O}(\Lambda^8)$, and the Majorana phases of
CP violation are not subject to the $\Lambda$-expansion. Terrestrial matter
effects on this new parametrization in realistic long-baseline neutrino
oscillation experiments are briefly discussed. Some comments are also given on
the possible relation between $\Lambda$ and a relatively weak hierarchy of
neutrino masses.
