rhoPronunciation: /rəʊ/Definition of rho noun (plural rhos) the seventeenth letter of the Greek alphabet (Ρ, ρ), transliterated as ‘r’ or (when written with a rough breathing) ‘rh’. (Rho) [followed by Latin genitive] Astronomy the seventeenth star in a constellation:Rho Cassiopeiae symbol (ρ) density. (ρ) Spearman’s correlation coefficient. Origin:Greekrho in other Oxford dictionaries Definition of rho in the US English dictionary