Capsaicin
The compound that makes chillies hot, concentrated in the white pith around the seeds.
Also: chilli heat compound, capsaicinoid, dihydrocapsaicin
Capsaicin is the chief member of a family of capsaicinoids made by chilli plants, probably as a defence against fungi and mammals, though birds, which lack the receptor response, spread the seeds. It binds to TRPV1, the heat receptor, and triggers a burning feeling even without a rise in temperature. It does not dissolve in water, but does in fat, alcohol and, through casein, milk. The highest levels are in the placenta, the pith, not the seeds themselves. Capsaicin is used in pepper spray and topical creams. Wash hands thoroughly after cutting chillies, and avoid touching the eyes.