Nearest

Nearest[list, x]: returns the one item in list that is nearest to x.
Nearest[list, x, n]: returns the n nearest items.
Nearest[list, x, {n, r}]: returns up to n nearest items that are not farther from x than r.
Nearest[{$p_1$ -> $q_1$, $p_2$ -> $q_2$, ...}, x]: returns $q_1$, $q_2$, ... but measures the distances using $p_1$, $p_2$, ...
Nearest[{$p_1$, $p_2$, ...} -> {$q_1$, $q_2$, ...}, x]: returns $q_1$, $q_2$, ... but measures the distances using $p_1$, $p_2$, ...

Return all items within a distance of 5:

- Nearest[{5, 2.5, 10, 11, 15, 8.5, 14}, 12, {All, 5}]
- Nearest[{Blue -> "blue", White -> "white", Red -> "red", Green -> "green"}, {Orange, Gray}]
- Nearest[{{0, 1}, {1, 2}, {2, 3}} -> {a, b, c}, {1.1, 2}]