Product identity (PI) refers to any terminology or content that is specifically and explicitly barred from reproduction under the Open Game License, as opposed to Open Game Content, which may be redistributed under said license. Product identity typically includes, but is not limited to, all trademarks and copyrighted terms, artwork and illustrations, and other design elements such as trade dress and logos.
The term closed content is occasionally used as a synonym for Product Identity, usually by explicitly distinguish the open content in a work from the non-open content. The term "closed content" isn't used or defined in the OGL itself, though the meaning is naturally inferred from context.
Some publishers explicitly grant a limited use of terms that fall under the umbrella of Product Identity. For example, several publishers grant a limited license to use PI terms that appear in the names of specific OGC elements such as feats, spells, or magic items; in these cases, OGL-compliant works can use the full names of the relevant OGC elements (including the PI referenced in these names), but cannot use that PI in any other context.
Dungeons & Dragons/d20 System Product Identity[]
The following items are designated Product Identity, as defined in Section 1(e) of the Open Game License Version 1.0a, and are subject to the conditions set forth in Section 7 of the OGL, and are not Open Content: Dungeons & Dragons, D&D, Player’s Handbook, Dungeon Master, Monster Manual, d20 System, Wizards of the Coast, d20 (when used as a trademark), Forgotten Realms, Faerûn, proper names (including those used in the names of spells or items), places, Red Wizard of Thay, the City of Union, Heroic Domains of Ysgard, Ever-Changing Chaos of Limbo, Windswept Depths of Pandemonium, Infinite Layers of the Abyss, Tarterian Depths of Carceri, Gray Waste of Hades, Bleak Eternity of Gehenna, Nine Hells of Baator, Infernal Battlefield of Acheron, Clockwork Nirvana of Mechanus, Peaceable Kingdoms of Arcadia, Seven Mounting Heavens of Celestia, Twin Paradises of Bytopia, Blessed Fields of Elysium, Wilderness of the Beastlands, Olympian Glades of Arborea, Concordant Domain of the Outlands, Sigil, Lady of Pain, Book of Exalted Deeds, Book of Vile Darkness, beholder, gauth, carrion crawler, tanar’ri, baatezu, displacer beast, githyanki, githzerai, mind flayer, illithid, umber hulk, yuan-ti.
Pathfinder Product Identity[]
The following items are hereby identified as Product Identity, as defined in the Open Game License 1.0a, Section 1(e), and are not Open Content: All trademarks, registered trademarks, proper names (characters, deities, etc.), dialogue, plots, storylines, locations, characters, artworks, and trade dress. (Elements that have previously been designated as Open Game Content are not included in this declaration.)
13th Age Product Identity[]
The following items are hereby identified as Product Identity, as defined in the Open Game License 1.0a, Section 1(e), and are not Open Content: All trademarks, registered trademarks, proper names (characters, icons, etc.), dialogue, plots, storylines, locations, characters, artworks, and trade dress. (Elements that have previously been designated as Open Game Content are not included in this declaration.)