Ghee
Clarified butter, cooked until the water and milk solids separate out; nutty and suited to high heat.
Ghee is sold in many supermarkets and reliably in Asian, South Asian or Middle Eastern grocers and health food shops. Clarified butter is essentially the same thing, just cooked less long and therefore less nutty.
Substitute:
Clarified butter replaces ghee readily, though it tastes milder. To make ghee yourself, simmer unsalted butter gently until the milk solids on the bottom turn light brown, then strain.