Etrog is the yellow citron or Citrus medica used by Jews during the week-long holiday of Sukkot as one of the four species. Together with the lulav, hadass, and aravah, the etrog is taken in hand and held or waved during specific portions of the holiday prayers. Special care is often given to selecting an etrog for the performance of the Sukkot holiday rituals.

The citron, historically cedrate, is a large fragrant citrus fruit with a thick rind. It is said to resemble a 'huge, rough lemon'. It is one of the original citrus fruits from which all other citrus types developed through natural hybrid speciation or artificial hybridization. Though citron cultivars take on a wide variety of physical forms, they are all closely related genetically. It is used in Asian and Mediterranean cuisine, traditional medicines, perfume, and religious rituals and offerings. Hybrids of citrons with other citrus are commercially more prominent, notably lemons and many limes.