Naevus (plural: naevi) refers to a common type of skin lesion, often known as a mole, that arises from melanocytes, the pigment-producing cells in the skin. Naevi can be classified into several types based on their appearance and depth in the skin. Common types include junctional naevi, which are flat and found at the junction between the epidermis and dermis; compound naevi, which are raised and extend into the dermis; and intradermal naevi, which are entirely within the dermis and tend to be more elevated. Additionally, there are congenital naevi, present at birth, and acquired naevi, which develop over time, often in response to sun exposure.

The causes of naevi are multifaceted. They often result from genetic factors, as certain genes can predispose individuals to develop moles. Environmental factors, particularly ultraviolet (UV) radiation from sun exposure, also play a significant role in the development and increase of naevi, especially in the case of acquired naevi. While most naevi are benign and require no treatment, any changes in size, shape, or color should be evaluated by a healthcare professional to rule out skin cancer, including melanoma.