Etrog (Hebrew: אֶתְרוֹג) refers to the yellow citron or Citrus medica used by Jews on the week-long holiday of Sukkot.
While in modern Hebrew this is the name for any variety of citron, its English usage applies to those varieties and specimens used as one of the Four Species.
Read more about Etrog: Etymology, Biblical References, Size and Shape, Color and Texture, Pitam (Pitom), Purity, Customs