Infinity

Infinity or ∞ is a symbol that represents a positive infinite quantity.

Examples

Use in limits:

Limit[1/x, x -> 0, Direction -> -1]
(* Infinity *)

In sums:

Sum[1/n^2, {n, 1, Infinity}]
(* Pi^2/6 *)

Please visit the official Wolfram Language Reference for more details.