The spiciness of peppers is call pepper pungency. Pepper pungency is based on the Scoville Heat Scale whereas the mildest pepper, bell peppers, have 0 Scoville Heat Units (SHU) and the current Guinness Book of World Records record holder for the spiciest pepper is the Carolina Reaper has a max SHU of 2,200,000.
We consider peppers that have between 100,001 - 500,000 SHU as being in the hot range. These peppers reside in the upper limit of what most people can tolerate without having extreme discomfort and the dreaded hiccups that some people experience when eating super hot peppers. If you regularly eat super hot peppers, the peppers in our extra hot category, you may find this category to be quite mild as your tolerance to capsaicin has built beyond this heat level.