O

O[x]^n represents a term of order x^n in power series, indicating omitted higher-order terms.

Examples

In series expansion:

Series[Sin[x], {x, 0, 3}]
(* x - x^3/6 + O[x]^4 *)

Remove O terms with Normal:

Normal[Series[Exp[x], {x, 0, 3}]]
(* 1 + x + x^2/2 + x^3/6 *)

Arithmetic with O:

(1 + O[x]^2) * (x + O[x]^3)
(* x + O[x]^2 *)

Please visit the official Wolfram Language Reference for more details.