■ combining form
1》 straight; rectangular; upright: orthodontics.
↘correct: orthoepy.
2》 Chemistry denoting substitution at two adjacent carbon atoms in a benzene ring: orthodichlorobenzene. Compare with meta- and para- 1 .
3》 Chemistry denoting a compound from which a meta -compound is formed by dehydration: orthophosphoric acid.
Origin
from Gk orthos 'straight, right'.