A mix of all of the alcohol in your house/apartment/living quarters into one drink.
It must include the supply of alcohol from all members living in the same place, not just one person.
Tim has Jack, vanilla vodka, Kahlua, and a bottle of Merlot. Jay has citron vodka, Sapphire gin, triple sec, and a couple bottles of Rolling Rock beer. Tim and Jay live in the same college dorm suite happily.
A Diffenheady is a drink of ALL of the above, preferably in equal distribution.