Tadka (noun.) is the Hindi translation of tempering or seasoning. Indian cuisine involves the use of whole herbs (cardamom (ilaichi), cinnamon (dalchini) and cloves (lavang)) and other spices or powdered herbs that are tossed in oil for tempering. You can either use oil or creamy butter to temper.
A North Indian tadka dish is mostly tempered in Ghee (clarified butter) that helps to extract and retain the flavours and aroma of herbs and adds a unique taste to the dish. In certain dishes, you can boil lentils (or vegetables) and temper with ginger-garlic (adrak-lehsun) paste, chopped tomatoes and onion to get a unique fresh flavour.