A fresh red pepper, either a long red chili or a red bell pepper: the ripe form of a pepper that is also sold green. Usually cut up and stir-fried with meat. Dried red chilies are a different ingredient.
This dish needs red and green peppers stir-fried together.
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红椒 covers any red-colored capsicum, sweet bell peppers as well as red chilies. Context (or the dish) tells you which. For specifically spicy, 辣椒 (làjiāo) is clearer.