Normal

Normal[expr] converts expr to a normal expression from a variety of special forms, such as SeriesData, SparseArray, etc.

Examples

Convert series to polynomial:

Normal[Series[Sin[x], {x, 0, 5}]]
(* x - x^3/6 + x^5/120 *)

Convert sparse array:

Normal[SparseArray[{1 -> a, 3 -> b}, 4]]
(* {a, 0, b, 0} *)

Please visit the official Wolfram Language Reference for more details.