Japanese pepper. 山椒, さんしょう (Zanthoxylum piperitum)
Sansho is not related to black pepper (kosho), but the berries are from the Japanese prickly ash shrub. The green pods are ground to a powder without the berries, and then dried (the berries are not used as they are quite bitter). Sansho is usually bought ground, as the flavor keeps a long time.
[Sansho. Photo Ad Blankestijn]
Sansho possesses in fact an aroma similar to citrus and mint. It has also been compared to "lemony aniseed" (World Food Japan by John Ashburne). It is aromatic rather than hot.
Sansho is used for grilled eel (unagi no kabayaki), yakitori and kushikatsu. With its citrus taste it serves to refresh the palate after oily dishes, making its use very different from pepper in the western kitchen. It is also used in udon, nabemono, and on some types of white fish (replacing its sister product shichimi togarashi).
Sansho is also one of the ingredients of Shichimi togarashi. The pods are also added to tsukudani.