Etrog: which fruit?
The Four Species One of the mitzvot of the festival is the taking of four species of plants. Only the palm branch and the willow are named in the Torah. The Sages identified the precise species of the other two based on their description in the Torah. “The fruit of a pleasant tree [peri etz hadar]” (Leviticus 23:40); Rabbi Hiyya taught: This is a tree the taste of whose tree and fruit is the same;28 this is the etrog. “Pleasant [hadar]” (Leviticus 23:40); ben Azzai says: A fruit that resides [hadar] on its tree from one year to the next. Akiles the convert translated it as hidor [Greek for water], as it grows on all types of water.29 “Branches [kappot] of date palms” (Leviticus 23:40); Rabbi Tarfon says: Bound [kafut]; if its leaves were separated, one must bind them together. “And a bough of a leafy tree” (Leviticus 23:40); whose branches cover its tree,30 you must say that this is the myrtle [hadas].31 (Vayikra Rabba [Margaliyot], Parasha 30)
Steinsaltz, Rabbi Adin Even-Israel. A Concise Guide to the Sages: An Overview of Jewish Wisdom (p. 376). Koren Publishers Jerusalem Ltd. Kindle Edition.