Prism is a refractive element that rotates the image of an object. The cross-section of a prism is triangular and the rotation of the image is away from the base (or toward the apex).
Any lens with a prescription power has some degree of prism built into it, so we could say that, technically, any pair of prescription glasses are "prism glasses." The prescribing doctor is responsible for managing the prism so that the glasses not only provide the desired clarity, but are comfortable as well. Usually this is very straightforward and is taken care of when prescription lenses are made specifically for you. (Over the counter "readers" cannot make this personal compensation and can cause problems when used for sustained near tasks.).
There are various reasons that a doctor may choose to manipulate the prism as a separate part of the prescription. Remember that the prism rotates the image away from the base of the prism so the doctor must specify the direction of the base as well as the amount of prism. The prism may be specified for one eye or for both eyes. Sometimes doctors may label this specified prism as "compensatory" or "therapeutic," but there is no generally accepted criteria to distinguish these terms.
Although prescribing prism can be very helpful, prisms are not without their difficulties. The basic description of what a prism is may seem very straightforward, but there are a number of "side effects" which may diminish the benefits especially for larger amounts of prism. For one thing, the effect of the prism is not uniform across the field of vision and this can create various distortions.
Optical transformations provided by lenses are an important part of vision therapy. There are several procedures that can make use of prisms of various kinds and these are often mounted in glasses frames. These prism glasses may shift the visual environment and require the person to adapt to the visual change through visually directed motor activities. Another pair of prism glasses using a different arrangement of prisms may help a person see how they are aligned or not aligned on a particular task or how consistently the two eyes continue to work together. Prisms used in these ways for visual training/vision therapy are not typically worn as an habitual prescription, they are instead used only during therapy activities under supervision of their optometrist and/or vision therapist.
When people suffer a loss of part of the visual field due to some trauma or disease, prisms are sometimes used to help them be more aware of the missing part of the field. These may be a fairly simple design that moves the whole field slightly to increase awareness, or it may be a very specialized type of "sighting" prism that covers only part of the field. This can allow the person to monitor the blind field for obstructions or oncoming objects quickly without turning the head or eyes and losing track of straight ahead.