Prism is a box on the rx prescription form that is not normally filled in. Occasionally, when the two eyes are not properly aligned and looking directly at the same thing, PRISM can be ground into the lenses in order to re-align them. This can occur with strabismus (ie - an eye turn) or in situations where the eyes are properly aligned but are under a tremendous amount of strain in order to keep them aligned.

For example, "OD 1/2 BO 1/4 BD and OS 1/2 BO" means that your right eye has 0.5 base out prism and 0.25 base down, and left eye has 0.5 base out prism.