plantain
A large, starchy banana that is only eaten cooked; firm and potato-like when green, soft and sweetish once the skin turns black.
Plantains are sold in African, Asian and Latin American grocers and increasingly in larger supermarkets alongside other tropical fruit. Green ones ripen at room temperature within several days to a week if a recipe calls for ripe, black-skinned fruit.
Substitute:
There is no good substitute, because dessert bananas fall apart when fried and are much sweeter. For dishes using ripe plantain, very firm, slightly underripe bananas will do at a pinch, though they turn softer and taste more of banana.