A hydrophobic molecule is a molecule that won't combine with water. The word "hydrophobic" describes this water-repelling characteristic; "hydro" means "water" in Greek, and "phobic" derives from "phobos," which means "fearing." Though these molecules don't literally fear water, they do not mix with water molecules.
Technically speaking, a hydrophobic molecule is low in polarity, either as a whole or just in part. These molecules have very few, if any, hydrogen acceptors or donors. Because water molecules have two atoms of hydrogen, the two molecule types are not compatible. Additionally, the long chains of carbon that are typically found in hydrophobic molecules also do not interact with water.
This phenomenon can be observed in fat molecules, which are nonpolar. If a chef pours melted butter into hot water, the butter will clump together and stay liquid until both the water and the butter have cooled enough to allow the butter to solidify. In both states, liquid and solid, the fatty butter will stay clumped together and separate from the water because of the hydrophobic nature of fat molecules.
Hydrophobic molecules can also be known as lipophilic. This term can also be remembered by looking at the Greek roots for the word. "Lipos" means "fat," and "philia" means "bonding."