Chilli peppers, also spelled chile or chili (from Classical Nahuatl chīlli [ˈt͡ʃiːlːi]) and known as hot peppers, are varieties of berry-fruit plants from the genus Capsicum, which are members of the nightshade family Solanaceae, cultivated for their pungency. Chilli peppers are widely used in many cuisines as a spice to add "heat" to dishes. Capsaicin and the related capsaicinoids give chillies their intensity when ingested or applied topically. Chilli peppers exhibit a range of heat and flavours. This diversity is the reason behind the availability of different types of chilli powder, each offering its own taste and heat level.

Chilli peppers originated in Central or South America and were first cultivated in Mexico. European explorers brought chillies back to the Old World in the late 16th century as part of the Columbian Exchange, which led to the cultivation of multiple varieties across the world for food and traditional medicine. Five Capsicum species have been widely cultivated: annuum, baccatum, chinense, frutescens, and pubescens.

The substances that give chilllies their pungency (spicy heat) when ingested or applied topically are capsaicin (8-methyl-N-vanillyl-6-nonenamide) and several related chemicals, collectively called capsaicinoids. Pure capsaicin is a hydrophobic, colourless, odourless, and crystalline-to-waxy solid at room temperature. The quantity of capsaicin varies by variety, and depends on growing conditions. Water-stressed peppers usually produce stronger fruits. When a habanero plant is stressed, for example by shortage of water, the concentration of capsaicin increases in some parts of the fruit.

When peppers are consumed by mammals such as humans, capsaicin binds with pain receptors in the mouth and throat, potentially evoking pain via spinal relays to the brainstem and thalamus where heat and discomfort are perceived. However, birds are unable to perceive the hotness and so they can eat some of the hottest peppers. The intensity of the "heat" of chillies is commonly reported in Scoville heat units (SHU), invented by American pharmacist Wilbur Scoville in 1912. Historically, it was a measure of the dilution of an amount of chilli extract added to sugar syrup before its heat becomes undetectable to a panel of tasters; the more it has to be diluted to be undetectable, the more powerful the variety, and therefore the higher the rating. Since the 1980s, spice heat has been assessed quantitatively by high-performance liquid chromatography (HPLC), which measures the concentration of heat-producing capsaicinoids, typically with capsaicin content as the main measure