Prefix denoting right, straight, correct.
Ortho-, from Gk. orthos "set upright, straight, true, correct, regular;" cf. L. arduus "high, steep," O.Ir. ard "high;" also cognate with Av. ərəduua-, as below; from PIE *eredh- "high."
Ardâ-, from Av. ərəduua- "upright, erect, risen;" cf. Skt. ūrdhvá- "upright, tending upward, high;" Gk. ortho, as above.
Fr.: eau ortho
The → water molecule in which the → nuclear spin of the constituent → hydrogen atoms are → parallel (→ orthohydrogen). In astrophysics the ratio between ortho- and → para-water is used to determine temperatures in → interstellar medium.