Allspice (aka Pimento) is not a mixture of spices, it is the dried fruit of the Pimenta dioica plant. Christopher Columbus was seeking pepper in the Caribbean, and he thought allspice was it. He brought it back to Spain, where it got the name "pimienta," (Spanish for pepper).
The name "allspice" was coined by the English, who thought it combined the flavour of several spices, such as cloves, pepper, and even cinnamon and nutmeg. The Allspice fruit are picked when green and unripe, and are then sun dried. When dry they are brown and look like peppercorns. Allspice has a complex aroma, with a taste similar to a combination of cinnamon and cloves, but hotter and more peppery. It is used in curry powders, sauces, meat dishes, pickling, soups and cakes.