Pronounced "lee-sion" with the emphasis on the "lee," a lesion can be almost any abnormal change involving any tissue or organ due to disease or injury. There are numerous types of lesions with different naming classifications.
Lesions can be categorized according to whether or not they are caused by cancer. A benign lesion is non-cancerous whereas a malignant lesion is cancerous. For example, a biopsy of a skin lesion may prove it to be benign or malignant, or evolving into a malignant lesion (called a premalignant lesion).
Lesions can be defined according to the patterns they form. For example, a bull's-eye or target lesion is one that looks like the bull's eye on a target. (In an X-ray of the duodenum, a bull's-eye lesion can represent a tumor with an ulcer (crater) in the center.) A coin lesion is a round shadow resembling a coin on a chest X-ray. It, too, is usually due to a tumor.
Lesions can also be categorized by their size. A gross lesion is one that can be seen with the naked eye. A microscopic or histologic lesion requires the magnification of a microscope to be seen. The basis of sickle cell disease is a molecular lesion, one that is not even visible with a microscope but is only detectable on the molecular (protein or DNA) level.
Location is another basis for naming lesions. In neurology, a central lesion involves the brain or spinal cord, i.e., the central nervous system. A peripheral lesion involves the nerves away from the spinal cord and does not involve the central nervous system.
There is a virtually endless assortment of lesions in medicine: primary lesions, secondary lesions, impaction lesions, indiscriminate lesions, irritative lesions, etc. Many are named for people including the Armanni-Ebstein lesion, a Bankart lesion, a Blumenthal lesion, and so on.
The word "lesion" comes from the Latin noun "laesio" meaning "an attack or injury" which is related in Latin to the verb "laedere" = "to hurt, strike or wound."
To translate medical terms into everyday English and dispel the mystery which may surround them, visit MedicineNet's Medical Dictionary MedTerms.com. There you will find numerous entries, such as this one on "lesion."