Full Idea
Aristotle categorised relations as accidents - Socrates's whiteness, the sphericity of this ball - entities dependent on substances. Relations are not substances, so they must be, if anything at all, accidents.
Clarification
'Accidents' are non-essential features of things
Gist of Idea
Aristotle said relations are not substances, so (if they exist) they must be accidents
Source
report of Aristotle (Categories [c.331 BCE], §7) by John Heil - Relations 'Historical'
Book Reference
'Routledge Companion to Metaphysics', ed/tr. Le Poidevin/Simons etc [Routledge 2012], p.312
A Reaction
Heil says this thought encouraged anti-realist views of relations, which became the norm until Russell.