Gid hanasheh (Hebrew: גִּיד הַנָּשֶׁה) is the term for sciatic nerve in Judaism. It may not be eaten by Jews according to Halacha (Jewish Law). The laws of prohibition regarding the gid hanasheh are found in Masechet Chullin (Hullin) Perek 7.
Removal
The removal of the gid hanasheh and chelev (other forbidden fats) is called nikkur. Since it is labor intensive to remove all the forbidden parts of the hindquarters of an animal, the entire hindquarters are usually sold to the non-kosher market outside of Israel and a few other markets with sufficient Jewish populations to justify the expense.
References
Gid hanasheh Wikipedia(Text) CC BY-SA