Fresh cilantro
The fresh leaves of the coriander plant, with a bright, lemony, slightly soapy aroma.
Cilantro sits in the herb section of many supermarkets, and Turkish, Middle Eastern or Asian grocers reliably carry it, usually in large, inexpensive bunches. It wilts quickly but keeps for several days with the stems standing in a glass of water like a bouquet, or wrapped in a damp cloth in the fridge.
Substitute:
There is no real substitute: flat-leaf parsley brings colour and freshness but not the distinctive lemony aroma, and dried cilantro tastes of almost nothing.