When the skin ulcerates and epidermis is stripped away and lost, it is known as:

The process being described occurs when the skin's protective layers, specifically the epidermis, are compromised. This condition is known as denudation. Denudation refers to the stripping away of skin or mucous membranes, revealing underlying tissues. In the context of skin, this can lead to ulcers or areas where the protective barrier is lost, making the skin more vulnerable to infection and other complications.

Other terms listed have distinct meanings that differentiate them from denudation: epilation refers specifically to the removal of hair from the skin, erythema indicates redness of the skin typically caused by inflammation or irritation, and desquamation describes the shedding of the outer layer of skin but does not necessarily imply the underlying tissues have been exposed. Thus, denudation accurately captures the full extent of epidermis loss as described in the question.