Lulav is a ripe, green, closed frond of the date palm tree. It is one of the Four Species used in the daily prayer services during the Jewish holiday of Sukkot. The other species are the hadass (myrtle), aravah (willow), and etrog (citron). Each type of plant represents different parts of your body because it shows that you worship God with all of your body.
To qualify for use as one of the Four Species, the lulav must be ramrod straight, with whole leaves that lay closely together, and not be bent or broken at the top.
The term Lulav also refers to the lulav in combination with two of the other species—thearavah and the hadass—that are bound together to perform the mitzvah of waving the lulav. These three species are held in one hand while the etrog is held in the other. The user brings his or her hands together and waves the species in all four directions, plus up and down, to attest to God's mastery over all of creation. This ritual also symbolically voices a prayer for adequate rainfall over all the earth's vegetation in the coming year. (See Four Species for the complete description and symbolism of the waving ceremony.)
The biblical reference to the four species in Sukkot can be found in Leviticus Chapter 23, verse 40. The etrog is referred to as "Citrus fruit" (Etz Hadar), and the Lulav is referred to as "Palm branches" (Kapot t'marim).