In proceeding to define the term, Aristotle says that οὐσία is used in four senses if not more:  the essential nature (τὸ τί ἦν εἶναι), the universal (τὸ καθόλον) the genus, and a fourth the subject (τὸ ὑποκείμενον).  Under, this fourth sense he proceeds to discuss the application of the term οὐσια to the matter, the form, and the resulting whole.  Without going further we may see that the use of the word in philosophy was full of difficulty and ambiguity.