 The technical term ‘real essence’ is introduced into the philosophical lexicon by the English philosopher John Locke (1632–1704) in his An Essay Concerning Human  Understanding[1]  (hereafter “Essay”) that was first published in London, in December of 1689. In order to understand a great many of Locke’s arguments in Books III and IV of the Essay, it is important to understand what a real essence is. Thus, the purpose of this entry is to explain the various interpretations of ‘real essence’ that are in the current literature. Another reason why this matters is that the concept of ‘real essence’ (or variations of it) has been at the center of much of the philosophical debate over natural kind realism since the time of Locke, so some of this entry can serve as an historical introduction to that debate as well.