Full Idea
The essence of a potency involves a relation to something else; if inertial mass is a potency then its essence involves a relation to a stimulus property (impressed force) and a manifestation property (acceleration).
Gist of Idea
The essence of a potency involves relations, e.g. mass, to impressed force and acceleration
Source
Alexander Bird (Nature's Metaphysics [2007], 5.3.3)
Book Reference
Bird,Alexander: 'Nature's Metaphysics' [OUP 2007], p.107
A Reaction
It doesn't seem quite right to say that the relations are part of the essence, if they might not occur, but some other relations might happen in their place. An essence is what makes a relation possible (like being good-looking).