Here are the definitions of the symbols/abbreviations used in a typical contact lens prescription:
Eye - Which eyes the prescription line item is for.
OD - Right Eye
OS - Left Eye
OU - Both Eyes
SPH - Sphere (this can also be referred to as Power or PWR)
BC - Base Curve (usually a number between 8 and 10)
DIA - Diameter (usually a number between 13 and 15)
Brand - The brand/type of contact lens that your doctor has fitted you for
Toric (Astigmatism correcting)
Some patients require lenses that are toric or astigmatism correcting. Prescriptions for these types of lenses will also contain two additional numbers that relate to the correction of the astigmatism. These two numbers are usually separated by an "X" and read as "times", they are indicated with the following symbols/abbreviations.
CYL - Cylinder (usually a number between -4.00 and +4.00)
AX - Axis (usually a number between 0 and 180)
Bifocal or Multifocal
A smaller minority of patients require lenses that are bifocal or multifocal. Prescriptions for these types of lenses will contain an additional number with the following symbol/abbreviation.
ADD - Also known as "Add Power" or "Extra Strength"
Additional Contact Lens Prescription Notes:
A contact lens prescription is not the same as an eyeglass prescription. In addition to the lens power, your contact lens prescription contains several other pieces of information related to the size of the lens.
Even the power of the lenses is generally not the same as in your eyeglass prescription. This is because:
The contact lens sits on the surface of your eye, while your eyeglasses sit about 12 mm in front of your eye
Regular soft contacts do not correct for your astigmatism, but your eye doctor will typically try to partially correct for it by changing the power of the lens.
Your prescription will also contain an expiration date. This is typically one year from the date that the contact lenses were fitted.
BC values range from about 8.0 to 10.0. The doctor fits the lens with the curvature most appropriate for your eye. Most lenses come in several different BC values. If your prescription does not contain a BC value, this is likely because your brand of lens only comes in one base curve.
BC values are a bit like clothes sizes - just because you are a BC 8.6 in one brand doesn't mean you will be the same in another brand.
Though not common, sometimes you can have a regular prescription for one eye, and a toric or bifocal prescription for your other eye. In this case you will not only have different parameters for each eye, but also usually a different brand/type of lens as well.