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You know what it is: it's quite possibly one of the most commonly used, and important adapters in the world. It goes by many names "1/8 to 1/4," "mini to standard," or "3.5mm to 6.3mm," but no matter what you call it, it will adapt single ended headphones with the small plug to fit into the big plug.
Oh yeah, and it's by AudioQuest, so you're getting high quality materials and build quality that won't turn the adapter into a weak spot in your system.