InverseSeries[s, x] takes the series s generated by Series, and gives a series for the inverse of the function represented by s.
InverseSeries performs "reversion" of series.
Given a series , InverseSeries[s, x] gives a series for such that .
InverseSeries can be applied to any SeriesData object with the appropriate structure, whether or not it has been generated by Series.
See The Mathematica Book: Section 3.6.4.
See also: Solve, InverseFunction.