Saffron is the name given to the stamens of the saffron crocus, Crocus sativus, which are harvested, dried, and used for cooking. Saffron has a pleasant spicy smell, and it contains a dye that colors food a distinctive deep golden colour. Safflower, Carthamus tinctorius, is often used as a less expensive substitute for saffron, as is turmeric, Curcuma longa, which mimics saffron's color well but not so much its flavor.
Soaked in water or alcohol, it can make an effective and edible food dye. It can be also be used in the dyeing of fabrics, although the orange/yellow colour is short lived, and hence other dyes may often be prefereable.
It seems that saffron is more expensive today in comparison to other spices,than in period (judging by usage) but that may reflect that other spices have gotten cheaper, rather than saffron more expensive.
In England during the 15th century to the 18th century, saffron was grown extensively in parts of Cambridgeshire and Essex. The Essex town of Saffron Walden got its name from being a market center for the saffron trade.