orth-
combining form
or ortho-1. straight : upright : vertical
    orthograde
2. perpendicular
    orthorhombic
3. correct : corrective
    orthodontia
4.
  a. hydrated or hydroxylated to the highest degree
      orthophosphoric acid
  b. involving substitution at or characterized by or having the relationship of two neighboring positions in the benzene ring
      ortho-xylene

orth-
combining form
or ortho-
Etymology: Middle English, from Middle French, straight, right, true, from Latin, from Greek, from orthos; akin to Gothic gawrisqan to bring fruit, Sanskrit ūrdhva upright, high, vardhate he increases
1.
 a. : straight : upright : vertical
  < orthoceras >
  < orthal >
  < orthograde >
  < orthosymmetric >
 b. : exact : parallel
  < orthodiagram >
  < ortho-cousin >
  < orthodome >
2. : correct : corrective
 < orthometry >
 < orthodontia >
3.
 a. : an acid in the highest hydrated or hydroxylated form known either in the free state or in salts or esters
  < orthoarsenic acid >
  < orthoformic acid >
  — compare meta- 4 c, pyr- 2a
 b.
  (1) : the relation of two neighboring positions in the benzene ring
  (2) ortho-, usually italic : a derivative in which two substituting groups occupy such positions — abbr. o-
   < ortho-xylene or o-xylene is 1,2-dimethyl-benzene >
   — compare meta- 4b, para- 2b
4. : derived from igneous rock — in the name of a metamorphic rock
 < orthogneiss >
 < orthosite >
— compare para-