Conjunctivitis is one of the most common nontraumatic eye complaint. It is a descriptive term referring to any inflammatory process that involves the conjunctiva; however, to most patients it is a diagnosis in its own right, often called "pink eye." Most causes of conjunctivitis are benign. On a cellular level, cellular infiltration and exudation characterize conjunctivitis.

Classification usually is based on cause, including viral, bacterial, fungal, parasitic, toxic, chlamydial, chemical, and allergic. It also can be based on age of occurrence or course of disease. Etiology often can be distinguished on clinical grounds. In keratoconjunctivitis an associated corneal involvement is present.