A unique item is one of four specific objects required to complete the game:
The discoveries page lists these items under the "Unique Items" section. Unique items are defined by having the field
oc_unique set to 1 in objects.c. While the unique items are also commonly referred to as invocation artifacts, they do not count as artifacts for wishing purposes, as the player is guaranteed to encounter them. Unique items are indestructible (barring bugs) and may not be placed in containers.
It is not possible to wish for any of the unique items.
Unique items versus artifacts
Starting with NetHack 3.0.0, all artifacts are also unique within a given game, but are not the same as unique items - they are specially named variants of their base item with additional benefits, and appear as named versions of those items when unidentified. For example, most games will have no more than one runesword or tsurugi, as those items never generate randomly and only appear when Stormbringer or the Tsurugi of Muramasa (respectively) are generated. Neither counts as a unique item - it is possible to wish for them, or find them in a bones file if the aforementioned artifacts already exist.
EvilHack adds a couple of other "one-per-game" items that serve as the base item for artifacts, and are never randomly generated (except when their associated artifact is generated):