Full Idea
A relation R is essential to x and y (in that order) iff Rxy holds at every world where x and y both exist.
Gist of Idea
A relation is essential to two items if it holds in every world where they exist
Source
Graeme Forbes (In Defense of Absolute Essentialism [1986], 2)
Book Reference
'Midwest Studs XI:Essentialism', ed/tr. French,Uehling,Wettstein [Minnesota 1986], p.4
A Reaction
I find this bizarre. Not only does this seem to me to have nothing whatever to do with essence, but also the relation might hold even though it is a purely contingent matter. All rabbits are a reasonable distance from the local star. Essence of rabbit?