OS stands for the left eye on a prescription, which is an abbreviation for a Latin phrase used in ophthalmology and optometry. These terms are a part of a historical convention that provides a universal language for medical professionals when documenting eye examination results and writing prescriptions. The abbreviations ensure clarity and precision, especially because the vision correction needed for each eye is often different. Understanding this Latin terminology is the first step in decoding your full eyeglass or contact lens prescription.
Defining OS Oculus Sinister
The abbreviation OS is derived from the Latin term Oculus Sinister, which translates directly to “left eye.” Oculus means “eye,” and sinister means “left.” This specific Latin phrase is used to denote all measurements and corrections pertaining to the vision of your left eye on a prescription form.
While the word “sinister” carries a negative connotation in modern English, its original Latin meaning was merely “left” or “on the left side.” Medical terminology frequently relies on these historical Latin roots to maintain a standardized, worldwide system of communication. This convention helps ensure that a prescription written anywhere in the world is understood by any optical lab or eye care professional.
When you look at your prescription, the numbers listed under the OS column indicate the required lens power, measured in diopters, for the left eye. A minus sign before the number, such as -2.00, signifies correction for nearsightedness (myopia). Conversely, a plus sign or no sign at all indicates a correction for farsightedness (hyperopia).
If there are additional values like cylinder (CYL) and axis, these specific corrections apply to the astigmatism in the left eye. The OS column details the precise optical adjustments needed to bring the left eye’s vision to its clearest potential.
The Full Set of Eye Abbreviations
The left eye designation (OS) is one part of a set of three Latin abbreviations traditionally used in eye care documentation. The companion term is OD, which stands for Oculus Dexter, translating to “right eye.” Dexter is the Latin word for “right” or “on the right side.”
The measurements listed under OD are solely for the right eye, detailing its spherical, cylindrical, and axial corrections. These two abbreviations, OS and OD, are typically listed separately on a prescription to allow for different corrective powers for each eye. This separation is important to prevent the wrong lens from being placed in the wrong side of the eyeglass frame.
The final abbreviation is OU, which stands for Oculus Uterque, meaning “both eyes.” This term is used less frequently on prescriptions, but when it appears, it indicates that the same vision correction values apply equally to both the right and left eyes. The use of OD, OS, and OU ensures that eye care professionals and optical dispensers can accurately interpret the required lens specifications.