The foundation upon which reverse osmosis stands depends on the nuts and bolts of the principles behind it. Voltaire wisely stated, “If you wish to converse with me, please define your terms.” To present the principles and operations of reverse osmosis (RO) in an understandable form, the terms associated with the technology are presented here in plain English. Moreover, to make sure that the essential and practical details and basic aspects of RO are easily understood, the concepts, units of expression, and pertinent nomenclature are also presented.