This legume is known as black gram or mungo bean. The product sold as black lentil is usually the whole urad bean, where the split bean, which has a white interior, is called the white lentil.
Urad dal chilka is split, but retains the skin and has a strong flavour. Urad dal, which is skinned and split, is creamy white and somewhat bland.
Urad dal is used daily in South Indian households to make the batters for vada, idli and dosa. It is also used for tempering, where the white lentils are added to the oil with spices to then pour over a dish.
Most of the recipes in this book call for the white lentil. This is what most Indian grocers will sell as urad dal.
Urad dal is available from Indian grocers or good health food shops. When purchasing urad dal in bulk or in a pre-packaged container, make sure that there is no evidence of moisture. They can be stored indefinitely in an airtight container in a cool, dry place.
Always rinse and sort lentils before cooking. Beans and lentils can have very small stones and debris that must be removed by hand.