Lens
A lens is any medium that transmits light and has an Index of Refraction and shape designed to bend light. Lenses come in many forms but there's a few critical ones that relate to EM.
Crystalline Lens
This is the lens inside your eyeballs. It is not actually the strongest part of your internal refractive state but it is the part that is adjustable. The ciliary muscle can flex this lens and change it's power. The ability to change the power of this lens to focus on near work is called accommodation. As you age, the crystalline lens thickens and is prone to presbyopia and cataracts.
Glasses or Contacts
If the refractive state of your eye causes myopia or hyperopia then you probably wear contacts or glasses. The EM method addresses the spherical and cylinder components of your prescription. The strength of your lenses are measured in diopters.