Nearest

Nearest[{elem1, elem2, ...}, x] gives the list of elem_i to which x is nearest.

Nearest[data, x, n] gives the n nearest elem_i to x.

Nearest[data] generates a NearestFunction[...] that can be applied repeatedly to different x.

Examples

Find nearest element:

Nearest[{1, 2, 5, 10}, 4]
(* {5} *)

Multiple nearest:

Nearest[{1, 2, 5, 10}, 4, 2]
(* {5, 2} *)

With points:

Nearest[{{0, 0}, {1, 0}, {0, 1}}, {0.5, 0.5}]
(* {{1, 0}} or {{0, 1}} *)

Please visit the official Wolfram Language Reference for more details.