saffron
Dried stigmas of a crocus flower that turn baked goods and dishes golden yellow and add a bitter-floral aroma.
Saffron is sold as threads or ground in the spice aisle of larger supermarkets, often kept near the till, and in delis and spice shops; threads are preferable because the powder is more often adulterated. There is no good substitute.
Substitute:
Turmeric or safflower match the yellow colour but not the flavour, and turmeric adds an earthy note of its own; the dish will look right but will not taste of saffron.