ˌprimo-ˈrationalMath.rare—1. [f. as prec. + rational, as derivative of prime ratio: see -o suffix1 1 and cf.politico-economic.] A quantity expressing a prime ratio: see prime a. 10 d. 1862De Morgan in Graves Life Sir W. Hamilton (1889) III. 576, I would rather use primo-rationals than differentials.