Coriander seed is a member of the parsley family. The seeds are used as a seasoning. When sprouted the leaves are referred to as cilantro or Chinese parsley. Coriander is available ground or whole.
The whole coriander seed is frequently used in Hispanic, Asian and some European cooking.
Whole coriander seed can be purchased in some well-stocked grocery stores. You can typically find it in bulk at Whole Foods Markets. You can also look for it in Hispanic and Asian markets. Whole coriander seed can be purchased online at Amazon.com.
When possible purchase whole coriander seed and grind your own. Coriander seed is soft and easy to grind in a mortar and pestle or in a small coffee grinder. Many times ground coriander is easier to find in the grocery store. Ground coriander loses its flavor pretty quickly so it should be replaced every couple of months if you use it a lot.
If your recipe calls for whole coriander seed but you only have ground coriander, then you can substitute about 3/4 - teaspoon ground per teaspoon whole seed. If your ground coriander has been in your spice rack for more than a couple of months then you can probably substitute 1 teaspoon ground for 1 teaspoon whole seed. If your ground coriander is pretty fresh then start with 1/2 teaspoon to 3/4 teaspoon.