Eruptive squamous atypia (ESA) is a distinct entity characterized by the presence of one or more scaly or crusted papules, plaques, or nodules that display a proliferation of well-differentiated, atypical keratinocytes on histopathology. ESA is also commonly referred to as eruptive keratoacanthoma (eruptive KA). However, this term is a misnomer as "keratoacanthoma" implies squamous cell carcinoma (SCC), whereas ESA represents reactive squamous proliferation.

ESA can be subcategorized into 2 morphologic types. The first type is focal koebnerizing ESA (FK-ESA), which appears as a hyperkeratotic papule or plaque that arises in areas of localized skin trauma, such as surgery, laser therapy, or tattoo application. The second type is diffuse ESA (D-ESA), which presents as multifocal ESA lesions, often on the arms or legs.

Both patterns may occur simultaneously. A skin biopsy should be used to confirm the diagnosis.