The aim of this paper is to clarify what the word hekaston signifies in Aristotle's theory of essence, as it is developed in his Metaphysics Z4-6 and 10-12. In Z4, Aristotle refers to "you, " an individual substance, as an example of hekaston, in order to explain the essence of hekaston. However, if we interpret the word hekaston to signify individual substance, it becomes difficult to understand the claim in Z6 that hekaston is to be identified with its essence. For the individual substance has essence as its essential aspect and accidents as its non-essential aspects. The alternative interpretation is that the word hekaston signifies form. But this interpretation is not appropriate to the context of Z4-6, where Aristotle does not take hylomorphism into consideration. So I suggest a third interpretation, according to which the word hekaston signifies an individual substance that is tentatively treated as that which has no accident. The advantage of this interpretation is that the word signifies such an individual substance throughout Z4-6. Furthermore, this interpretation helps us to understand the difference between the viewpoint of the first half of the theory of essence and that of the latter half.