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Definition of Ortho- (prefix)
Medically reviewed by Min Clinic Staff | Updated: January 2026
Ortho- (prefix): Prefix meaning straight or erect. From the Greek "orthos" that means just that: straight or erect. Examples of terms involving ortho- include orthodontics (straightening the teeth), orthopaedics (straightening the child), orthopnea (breathing easily only in an upright position), orthostatic (an upright posture), etc.