ortho-

Right angle, perpendicular.

In this sense, it is the angle that is "right" or straight rather than the line; orthogon is a right-angle triangle, orthocentric is a tetrahedron where all three pairs of opposite edges are perpendicular, orthoclastic is a crystal whose cleavage planes are at right angles.

Proper, righteous, pure.

E.g., orthorexia is an unhealthy fixation with healthy or righteous eating, an orthotaxon is a true taxon based biological relationships rather than form, orthology is speech which is proper, orthoquartzite is very pure quartz sandstone.

To correct or fix.

E.g., orthophotography is the geometrical correction of aerial photographs, orthoptics is the treatment of muscular irregularities of the eye, orthodontics is the correcting of misaligned teeth.

Same, from the same.

E.g., An orthocousin, unlike a cross-cousin, is on the same side of the family; orthogamy is when a plant's ovules are fertilized by pollen from the same plant; orthochronous is a transformation which preserves the direction of time.

Normal, ordinary.

E.g., orthocclusion is normal occlusion, orthodromic is when neurological impulses are passed in the normal direction, orthogravitropism is the ordinary effect of gravity on plants