10% milk fat (more exactly 1.6 lb per pre-mixed gallon, but that's simply a bizarre way of phrasing it), no more than half air by volume. 6-10% other dairy solids (lactose, whey).
Compare with the UK: at least 5% fat (no cows need be involved)
France requires 5% milkfat, Germany at least requires the 10% milk fat, but no further requirements.
Canada pretends to be at 10%, but if you add any flavoring at all that can go down to 8%.