We often use the term 'essence' in order to denote form. In natural substances, though, we must remember that 'essence' most properly denotes not form, but the substance which is composed of both matter and form. If essence was merely constituted by form within natural compounds, man would not be defined as a rational animal but only as a rational soul.
The reason that essence is more closely associated with form is because form is that which makes matter intelligible, meaning it is the act which determines some potency in real existence. This means that something is definable inasmuch as it is in act; inasmuch as it is determined by some form. But it is also within the analysis of definition that we matter's essential role. Again, to have a body is just part of what it means to be a man.