forming words, with the meanings:
straight, upright, vertical, or perpendicular
orthogonal
orthorhombic
correct or corrective
orthoptics
orthodontics
in chemistry, containing the highest possible number of hydroxyl groups or molecules of water
in chemistry, involving substitution at two neighbouring positions in the benzene ring: compare meta-, para-1
[Middle English via French and Latin from Greek, from orthos straight, right]