Dictionary Definition: 

“Tending to repel or fail to mix with water.”

Selected from OxfordDictionaries.com

Functional Definition: 

I’m sure you know what hydrophobicity is or have seen it in general chemistry or in the parking lot or kitchen when motor oil and canola oil don’t mix in a pan of water or in the rain.  This behavior of apparent repulsion explains the hydrophobic effect. Another way of saying this is that nonpolar molecules tend to aggregate in polar solutions. The nonpolar oils or organic molecules form a separate layer as they orient themselves as to decrease surface contact as much as possible with water. In many cases these organic molecules form protocells or micelles and this relationship has been used to hypothesize how cells first came to exist.