A rule stating that an elimination reaction will give as the major product the most stable alkene, that is, the alkene with the most highly substituted double bond.
For example, dehydrohalogenation of alkyl halide 2-Bromo-2-methylbutane can proceed in two ways to give two types of alkenes, of which only one is a major product.
The product outcome is primarily determined by the double-bond character that develops during the reaction's transition state. The reaction favours the product that leads to the formation of a highly substituted alkene, which is also considered the most stable.
The rule is named after 19th-century Russian chemist A. N. Zaitsev, who formulated it. The transliterated variants of Zaitsev's name are Zaitzev, Saytzeff, Saytseff, or Saytzev.
Zaitsev's rule is very important in understanding alkene regioselectivity, mainly in E2 (bimolecular) type of elimination reactions.
Related – Most and least substituted alkene (tutorial)