Full Idea
Moore characterises internal relations modally, as those essential to their relata. If a and b are related R-wise, and R is an internal relation, a and b could not fail to be so related; otherwise R is external.
Gist of Idea
A relation is internal if two things possessing the relation could not fail to be related
Source
report of G.E. Moore (External and Internal Relations [1919]) by John Heil - Relations 'Internal'
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.313
A Reaction
I don't think of Moore as an essentialist, but this fits the essentialist picture nicely, and is probably best paraphrased in terms of powers. Integers are the standard example of internal relations.