Tamarind paste
The sour, fruity pulp of tamarind pods, pressed or cooked down into a dark paste.
Tamarind paste is sold in Asian, Indian and Turkish grocers and increasingly in well-stocked supermarkets, either ready to use in a jar or as a pressed block that you soak in warm water and push through a sieve. Check the label, because concentrates are much stronger than plain paste and should be used more sparingly.
Substitute:
Lime juice with a little brown sugar supplies the sourness but not tamarind's fruity, dark, slightly caramel depth, so the dish will taste brighter and flatter.