What does "add" power in multifocal prescriptions refer to?

The term "add" power in multifocal prescriptions specifically refers to the additional magnifying power required for near vision. In multifocal lenses, particularly bifocals and progressives, the "add" power denotes the increase in lens power that facilitates sharp vision for close-up tasks, such as reading or sewing. This additional power is combined with the distance portion of the lens to allow the wearer to transition smoothly from seeing distant objects clearly to focusing on near objects.

Multifocal lenses incorporate this additive aspect so that individuals who need correction for both distance and near vision can have a functional solution without needing to switch between multiple pairs of glasses. Thus, when a prescription indicates an "add" power, it's specifically indicating how much more power is needed for near vision vs. distance vision.