Lobe: Part of an organ that appears to be separate in some way from the rest. A lobe may be demarcated from the rest of the organ by a fissure (crack), sulcus (groove), connective tissue or simply by its shape. For example, there are the frontal, parietal, temporal, and occipital lobes of the brain.
The word "lobe" from the Greek lobos meaning "a small rounded projection" long applied only to the familiar round projection at the base of the ear -- the earlobe. The lobes of the brain, thyroid, lungs, and liver were hardly mentioned in English until the 16th century.