Commonly Used Terms

O.D. stands for "Oculus Dexter", which is Latin for "right eye"
O.S. stands for "Oculus Sinister," which is Latin for "left eye"
D.V. "Distance vision" relates to far-sightedness — a condition where distant objects are seen clearly, yet objects close up are seen less clearly.
N.V. "Near vision" relates to near-sightedness — a condition where near objects are seen clearly, yet objects in the distance are seen less clearly.
SPH stands for "sphere", and corresponds to numbers that indicate the degree to which a patient is near-sighted or far-sighted
CYL means "cylinder", and corresponds to numbers that indicate the amount of astigmatism present in a patient's eyes. Astigmatism is an eye condition resulting from an irregular curvature in the eye, causing mild, moderate or severe blurred vision.
AXIS corresponds to numbers indicating the orientation of astigmatism in the eyes — how and where the irregular curvature occurs.
PRISM is a prescription element that's added if your eyes need help either working more effectively together, or staying in proper alignment.
PD stands for "Pupillary Distance", and measures the distance between your pupils (in millimeters). This measurement helps ensure your prescription is properly placed in your frames.
Seg Height is a measurement of where your lenses and frames sit relative to your pupils. It is important when fitting multifocal (bifocal, trifocal or progressive) lenses.
Looking at the prescription for eyeglasses, one will see numbers listed under the headings of OS and OD. They are Latin abbreviations; OS (oculus sinister) means the left eye and OD (oculus dextrus) means the right eye. Occasionally one will see a notation for OU, which means something involving both eyes. In general, the further away from zero the number on the prescription, the worse one's eyesight and the more vision correction one need. A plus sign in front of the number means and individual is farsighted and a minus sign means one is nearsighted. These numbers represent diopters, the unit used to measure the correction, or focusing power, of the lens the eye requires. Diopter is often abbreviated "D."
For example, if the prescription says -1.00, and individual has one diopter of nearsightedness. This is a fairly mild amount of nearsightedness. If an individual is -4.25, that means he/she has 4 and 1/4 diopters of nearsightedness. This is more nearsighted than -1.00, and requires stronger (thicker) lenses. Similarly, +1.00 would be a small amount of farsightedness and +5 would be more.
For people who have astigmatism, there will be three numbers in your prescription. The general form for writing these numbers is S x C x Axis
The S refers to the "spherical" portion of the prescription, which is the degree of nearsightedness or farsightedness discussed above.
The C refers to the "cylinder" or astigmatism, and can be a negative or a positive number. It measures in diopters the degree of astigmatism that and individual has. The bigger this number, the more astigmatism one has.
The Axis is a number anywhere between 0 and 180 degrees. It reveals the orientation of the astigmatism It is not enough to specify how much astigmatism there is; one has to know where the difference in curvature is taking place.
Here are two examples of what prescriptions for eyes with astigmatism could look like:
-2.00 +1.50 x 180
+3.50 +3.00 x 45
The first prescription means that the person has 2 diopters of nearsightedness with 1.5 diopters of astigmatism and an axis of 180 degrees.
The second prescription means that the person has 3.5 diopters of farsightedness, 3 diopters of astigmatism and an axis of 45 degrees.