Merriam-Webster Collegiate Dictionary - orth-

 or ortho-  combining form  Etymology: Greek, from orthos straight, right, true; akin to Sanskrit urdhva high, upright  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