What kind of chili is called phrik khi nu in Thai cuisine?

The chili called phrik khi nu in Thai cuisine is the bird's eye chili.

In Thai, phrik means chili, while khi nu literally means “mouse droppings,” a reference to the pepper’s small size and shape. It is also called Thai chili or bird’s eye chili in English. The small, intensely hot pods are used fresh or dried in curry pastes, dipping sauces, stir-fries, soups and spicy salads.

Phrik khi nu is not the same as a cayenne, serrano or habanero, although all are Capsicum peppers and can sometimes substitute for one another. Thai cooks often use it in nam phrik, a broad family of chili-based sauces and pastes. A related form, phrik khi nu suan, is even smaller and hotter.

Chili peppers were not native to Thailand. They originated in the Americas and reached Asia through Portuguese trade during the Columbian Exchange, probably in the 16th century. Thai cuisine later made them a defining source of heat.