One quantity stated in terms of another. A ratio survives scaling — it describes relation, not size.
Latin ratio (reckoning, calculation, reason, relation), from reri (to reckon, to think); a ratio names a relation between quantities that survives no matter how large or small either one becomes.
One quantity stated in terms of another. The root's reckoning is scale-proof by nature: a ratio describes the relation, not the size, which is why it survives being scaled up or down entirely.