As UK optometry and dispensing optics embrace the diagnosis and treatment of some eye diseases and contact lens complications, a thorough understanding of general pathology has become essential. This article, the first in a series to be published in Optometry in Practice, defines and characterises some of the terminology used in describing pathological processes and diseases and, where possible, an illustrative ophthalmic context is given. Future articles in the series will go into more detail about distinct pathological processes and their associated disorders.