1,193. Defining the Esrog, Hadassim and Aravos

Hilchos Shofar, Succah v’Lulav 7:2

When the Torah speaks of “the fruit of a beautiful tree,” it refers to the esrog. The "branch of covered trees" refers to the myrtle, whose leaves surround the branch so that there are three or more leaves in each ring. If two leaves are on the same level and the third leaf is slightly higher, it is not considered "covered." This is referred to as a wild myrtle.

Hilchos Shofar, Succah v’Lulav 7:3

"Willows of the stream" (arvei nachal) doesn’t mean any random plant that grows by a stream. Rather, it refers a particular species, whose name is “arvei nachal.” The leaf of this plant extends like a stream, its edges are smooth and its stem is red. This is called a willow. Most of the species grow near streams, which it’s why it’s called "willows of the stream" but even if it grew in the desert or on a mountain, it would be valid for use.