Definition of ortho- in English:

ortho-

combining form

  • 1Straight; rectangular; upright.

    ‘orthodontics’
    1. 1.1Right; correct.
      ‘orthoepy’
  • 2Chemistry
    Denoting substitution at two adjacent carbon atoms in a benzene ring, e.g., in 1, 2 positions.

    ‘orthodichlorobenzene’
    Compare with meta- and para-
  • 3Chemistry
    Denoting a compound from which a meta-compound is formed by dehydration.

    ‘orthophosphoric acid’

Origin

From Greek orthos straight, right.