Sichuan peppercorn is a spice produced from the husks of seeds of two species of the prickly ash shrub (Zanthoxylum), which is in the rue or citrus family. The pinkish-red husks around the seeds are used for the Sichuan peppercorn spice, while the inner black seed is discarded as it is too gritty and would be sand-like when eaten. Sichuan peppercorns can be used whole or ground into powder. The spice is one of the five ingredients that comprise five-spice powder (the others are star anise, fennel, clove, and cinnamon), and it's used in many savory Sichuan dishes.
The aroma of Sichuan peppercorn has been likened to lavender. On the tongue, the first taste is bitter, then numbing heat, followed by citrus. Its main claim to fame is the powerful numbing sensation it causes around the mouth. Sichuan peppercorns contain the molecule hydroxy-alpha-sanshool. Like capsaicin in chili peppers, it interacts with nerve cell receptors in your lips and mouth. It excites the touch sensors and the confusing sensation feels like numbness. The chemical triggers the receptors, causing them to fire a message to your brain that the area is being touched. With enough stimulation, it feels like the area has gone numb.