A thin, peppery, sour broth of tamarind and tomato, with cumin and garlic — poured over rice, or drunk from a cup.
What makes it rasam
Thin, not thick — a broth, not a dal.
Tamarind and tomato for sourness.
Black pepper, cumin and garlic, crushed rather than ground fine.
A tempering of mustard seed and curry leaves, and fresh coriander.
Where it comes from
Rasam comes near the end of a South Indian meal, mixed into the last of the rice, and it's what people make when someone has a cold. Its Tamil cousin milagu thanni — 'pepper water' — is widely agreed to be the ancestor of the Anglo-Indian mulligatawny.
How it varies
Mysore rasam
With some cooked dal and a roasted spice paste with coconut — fuller, and a little sweet.
Milagu rasam
Pepper rasam, heavy on black pepper, for colds.
Lemon rasam
Lemon juice instead of tamarind, added off the heat.