what is add in eye prescription

In an eye prescription, ADD stands for Addition, which is the additional correction needed for reading. It is used in bifocal glasses, reading glasses, or varifocal glasses. The ADD value is always a plus power, even if it appears without a plus sign. It represents the additional power over the distance prescription and is typically between +0.50 to +3.50. The ADD value may be written in the "Near Addition" space on the prescription, but it can also be written somewhere under or next to the distance values. The ADD value applies to both eyes and can sometimes be written only once on the prescription, even though the addition is required for both eyes.