Related:
* Elite NPCs

Obviously being literally a boss does not alone qualify for this. These may also exist even if no other battles are fought, though such cases are likely quite rare.

Battles against literal boss characters or other major antagonists that do not differ from other battles fought significantly (equally weak and such) this tag no longer conveys anything useful for them and should not be used for them (you might want to add a note about them why you think they do not deserve this, though).

Raid boss is an MMO variant of the end level boss (raid being a combination of multiple player groups/parties, usually established for the sole purpose of fighting the raid bosses).