Indexed

Indexed[expr, i] represents the component of expr with index i and formats as expr_i.

Indexed[expr, {i, j, ...}] uses multiple indices.

Examples

Indexed variable:

Indexed[x, 1]
(* x_1 displayed with subscript *)

Multi-index:

Indexed[A, {i, j}]
(* A_{i,j} *)

Please visit the official Wolfram Language Reference for more details.