To ensure your contact lenses are safe, comfortable and provide an optimum vision visit an ophthalmologist  before purchase. Keep your prescripton when buying a new pair of eye contacts. 

These are standard terms, abbreviations and measurements to write contact lens prescriptions as below:

OD: means oculus dexter, Latin for "right eye".

OS: means oculus sinister, Latin for "left eye". If both the left eye and the right eye have the same  diagnosis and prescription, the abbreviation "OU" or "ocular uniter" is used, meaning both eyes.

Sphere indicates the amount of lens power, measured in diopters (D), prescribed to correct nearsightedness or farsightedness. The spherical power of the lens is indicated with a plus sign (+) for hypermetropia and a minus sign (-) for myopia. 

Cylinder is a necessary parameter in choosing lenses for astigmatism correction. Indicates the number of diopters required for correction. It always has negative (-) values.

Axis it is also a parameter for toric lenses. It is expressed in degrees and includes values between 0 and 180 degrees. The axis indicates the orientation of the cylindrical diopter in the lenses to compensate astigmatism defects.

Base curve indicates the degree of curvature inside the lens. It is measured in millimeters and includes values between 8.3 and 10 mm. For maximum wearing comfort, the contact lens must match the base curve of the cornea.

Diameter indicates the distance, in millimeters, from one edge of the contact lens to the opposite side. It includes values between 13.8 mm and 14.5 mm. Crazy colored contact lenses have an even larger diameter, such as semi sclera -17 mm, sclera - 22 mm.