Sansho pepper
Ground seed husks of a Japanese shrub in the citrus family, not a true pepper. Fresh and lemony in aroma, with a light tingling, numbing heat on the tongue.
Sansho is the dried, ground seed husk of the Japanese pepper shrub, a close relative of Sichuan pepper; the hard black seeds are usually removed. It tastes bright, lemony and slightly resinous, with a tingle on the tongue and lips that numbs more than it burns. Because this aroma fades quickly, it is sprinkled over the finished dish at the table or just before serving, for example on grilled eel, rich meat dishes or broths, rather than cooked in for long. Ground sansho is sold in small tins at Asian grocers or by mail order; once opened, keep it cool and tightly closed and use it within a few months. It is also one of the components of the Japanese spice blend shichimi togarashi.
Substitute:
The closest substitute is finely ground Sichuan pepper, ideally freshly crushed in a mortar and sifted. It is earthier and woodier and numbs more strongly, but lacks the bright citrus note; a pinch of finely grated lemon or yuzu zest partly makes up for that. Black pepper is no substitute, since it burns rather than tingles.