The "Four Species, or Types of Agricultural Produce" which the Jew is commanded by the Torah to hold together and wave in all directions. The symbolism of this act, at least according to one opinion in the Talmud, is to show the supremacy of G-d, its Creator, over Nature.
The Four Species are the "Etrog," the Citron Fruit, the "Lulav," the branch of the date palm, "Hadasim," three Myrtle Twigs, and "Aravot," two Willow Branches.
Another symbolic interpretation of the "Four Species" is based on the assumption that "Taam," or Taste, represents knowledge of Torah while "Re-ach," or Aroma, represents the accomplishment of good deeds.
Thus, the "Etrog" which has good "Taam" and good "Re-ach," represents the individual who has much knowledge of Torah, and a treasure house of good deeds to his credit.
The "Lulav," associated with the date, has good taste, but little aroma, and represents the individual who is a Torah Scholar, but has only a few good deeds.
"Hadasim," symbolize the individual who has "Re-ach," but little "Taam," good deeds, but only a little knowledge of Torah.
The poor "Hadasim," though as creations of Hashem, are beautiful, have neither taste nor aroma. Thus, they represent individuals who have just a little Torah knowledge and very few good deeds to their names.
Thus, just like at the Pesach Seder Table, the Four Sons symbolize the Jewish People, which contains all types of individuals, so the act of holding the "Arba-ah Minim," the Four Species together, represents the unity of the Jewish people, and the fact that it is made up of individuals with all possible mixes of Torah knowledge and good deeds.

This definition is in context to Judaism. See more contextual defintions for "Arba-ah Minim".