Position

Position[expr, pattern] gives a list of the positions at which objects matching pattern appear in expr.

Examples

Find positions of an element:

Position[{a, b, c, b, a}, b]
(* {{2}, {4}} *)

Find positions in nested lists:

Position[{{1, 2}, {3, 1}}, 1]
(* {{1, 1}, {2, 2}} *)

Please visit the official Wolfram Language Reference for more details.