“A definition is an account, and every account has parts, and part of the account stands to part of the thing in just the same way that the whole account stands to the whole thing” (Aristotle 1034b20-22). This quote is how Aristotle defines a definition. So a definition is the statement of the essence of something. Defining something consists of starting with a genus and then breaking it down into species. A genus is a kind of a thing.
A species is a more specific kind of something that is within a genus. Aristotle notices that something cannot be defined by its material components because each component can infinitely be broken down into more and more components(Aristotle 1035b9). Aristotle states the example- a circle can not be defined in terms of semicircles because semicircles would then have to be defined by quarter circles and so on, but a semicircle can be defined in terms of a circle because a circle is the simplest(Aristotle 1035b9).
A genus is differentiated into species by dividing the genus into categories or certain types of the genus. Each of these categories or types within the genus are known as species. A species can also be a genus in that it too can be divided into specific categories or types. Those categories or types are also known as species, but can also be genus because they can further be divided into species. This pattern can go on infinitely.
A ball is a substance that can be categorized into each setting of play that a ball is used on. So a ball can be categorized into something that is used to score on a soccer field, football field, baseball field, tennis court, basketball court, pool, and golf course. Each of these fields uses a specific type of ball. A golf course uses a golf ball. The definition of a golf ball is the ball that is used on a golf course. The essence of a golf ball is a ball.
Metaphysics. Trans. Hugh Tredennick. 2 vols. Loeb Classical Library 271, 287. Harvard U. -Press, 1933-35