Everyone should have ajwain in their kitchen, and they should be using it all the time. It has a pleasant thyme aroma: clean, herbal, and with a bite like the best zaatar or spicy oregano. It’s absolutely ideal for any kind of spice rub on meat, north Indian curry, French soup, Mexican sauces, Ethiopian spiced butter, or Mediterranean stew. It’s not a familiar spice, but it’s common in Persian, Afghan, and northern Indian cooking.
This is a member of the carrot family, Apiaceae, along with a number of other common kitchen spices. It looks like cumin or caraway, but the seeds are smaller and fatter.
If you don’t have it and need a substitute, the closest taste is dried oregano. It’s available at a lot of Persian and Indian markets, though — or cheaply on Amazon. It’s meant to be antiflatulant … make of that what you will.