Star anise
The dried, star-shaped fruit of an evergreen tree, with a strong, sweetish aniseed and liquorice aroma.
Star anise is sold in the spice aisle of many supermarkets, especially around Christmas, and year-round and more cheaply in Asian or Turkish grocers. It is usually cooked whole and removed before serving, as one or two stars already give a strong flavour.
Substitute:
Aniseed comes closest in flavour, about half a teaspoon per star, but tastes flatter and lacks the slightly bitter warmth; fennel seeds are milder still.