Chilli
Also known as Capsicum annuum, Capsicum frutescens, chili, chile, peperoncino, guindilla, cayenne, Chilischote
The hot capsicums, distinct from the sweet paprika and pimentón made from the same genus. Capsaicin is fat-soluble, which is why heat behaves differently in a fatty sausage than it does on the tongue.
Chilli and paprika are the same genus and often the same species. What separates them is the cultivar and what the grower wanted: paprika and pimentón are made from capsicums bred for colour and sweetness with the heat mostly absent, while chilli means the ones bred to burn. A recipe that lists both is not being redundant; it is asking for colour from one and heat from the other. Confusing them, or substituting hot pimentón for sweet, is one of the more common ways to wreck a chorizo.
The burn comes from capsaicin and its relatives, concentrated not in the seeds, as the folklore has it, but in the pale placental tissue the seeds are attached to. Removing the seeds reduces heat mainly because it removes the membrane they cling to. Capsaicin binds the same receptor that responds to physical heat, which is why the sensation is genuinely one of burning rather than a taste.
The point that matters in charcuterie is that capsaicin is fat-soluble and not water-soluble. In a lean product the heat sits on the surface of the palate and arrives fast. In a fatty one, an nduja, a sobrasada, a well-marbled salami, the capsaicin distributes through the fat phase and is released slowly as the fat melts at mouth temperature, so the heat builds and lingers instead of striking. This is why a fatty chilli sausage can taste mild on the first bite and fierce by the third, and why judging the seasoning of a fresh mix by a fried test patty is more reliable than tasting the raw farce.
The traditions that use it heavily are Calabrian, above all in nduja and soppressata piccante, the Balearic sobrasada, the Spanish and Latin American chorizos picantes, the Hungarian and Balkan sausages, and the Asian cured meats. Northern European charcuterie uses it sparingly if at all.
Form matters as much as quantity. Whole dried chillies, flakes, and powders of the same weight deliver quite different heat, because the ratio of placental tissue to flesh changes with how the chilli was processed, and because commercial powders are often standardised by blending. Fresh chilli brings water into the mix on top of the heat. Where a formula gives a variety by name, Calabrian peperoncino, guajillo, cayenne, it is specifying a flavour as well as a heat level, and a generic substitution changes both.