Ajwain is a seed most commonly used in Indian cooking. The plant is similar to parsley. The seed is similar in smell to thyme but stronger and slightly musty. Ajwain originated in the Middle East, possibly in Egypt but is now primarily grown and used in India as well as in Iran, Egypt and Afghanistan.
Ajwain is added to Indian breads and used as a flavoring on biscuits as well as other snack foods. The seeds are also useful added to vegetables such as green beans and potatoes.
Typically this product is found in a store that specializes in Indian foods or you can find it online at Amazon.com: Ajika Organic Ajwain Seed, 2.2-Ounce
If you don't have ajwain and you prefer to find an alternative you can easily substitute equal amounts of:
Red Lentils with Paprika and Ajwain - Both Indian and Ethiopian cooks use ajwain when making lentils; this recipe is something of a hodgepodge of those two traditions.