A lesion is any abnormal tissue found on or in an organism, usually damaged by disease or trauma. Lesion is derived from the Latin word laesio which means injury. Lesions are caused by any process that damages tissues. A cancerous tumor is an example of a lesion, however the surrounding tissue damaged by a tumor is also a lesion. Trauma, including electrocution and chemical burns can also cause lesions. Certain diseases present lesions, for example the skin deformities caused by chicken pox. Lesions can also be caused by metabolic processes, like an ulcer or autoimmune activity, as in the case with many forms of arthritis.
Lesions are sometimes intentionally inflicted during neurosurgery, such as the carefully-placed brain lesion used to treat epilepsy and other brain disorders.
Note that lesions are not limited to animals or humans; damaged plants are said to have lesions.
Because the definition of lesion is so broad, the varieties of lesions are virtually endless. They are subsequently classified by their features. If a lesion is caused by cancer it will be classified as malignant versus benign. Lesions may be classified by the shape they form, as is the case with many ulcers, which can have a bullseye or 'target' appearance. Their size may be specified as gross or histologic depending on whether they are visible to the unaided eye or require a microscope to see.
An additional classification that is sometimes used is based on whether or not a lesion occupies space. A space occupying lesion, as the name suggests, occupies space and may impinge on nearby structures, whereas a non space occupying lesion is simply a hole in the tissue, e.g. a small area of the brain that has turned to fluid following a stroke.
Some lesions have specialized names, like the Gohn lesions in the lungs of tuberculosis victims. The characteristic skin lesions of a varicella-zoster virus (VZV) infection are called chickenpox. Lesions of the teeth are usually called dental caries.
Finally, lesions are often classified by their location. For example, compare a 'skin lesion' versus a 'brain lesion'.
