Nævus (Lat., 'a mole;' known popularly as mother-spot or birth-mark) is a congenital mark or growth strictly on a part of the skin. The most frequent form is the pigmentary nāvus, or mole. This may be simply a darker pigmentation of a circumscribed portion of skin; or the pigmented skin may be thickened and rough as well, and is often thickly covered with hair. Moles do not tend to increase, and do not need to be treated unless for the sake of appearance. In that case, removal by knife or destruction by caustics must be resorted to.
When the name is used without qualification, a vascular nāvus or overgrowth of capillary blood-vessels is generally meant, and the term is used of such abnormal growths in whatever organ or tissue they occur. The slightest form is sometimes called port-wine stain, and is sufficiently described by the name: there is just so much overgrowth as to produce a deep red discoloration, without appreciable swelling of the part affected. Frequently the abnormal tissue forms a distinct tumour, either in the skin, when it is of a dark red colour, or beneath it, when it may sometimes be recognised by a blue or purplish tinge. The most frequent situations of these vascular nāvi are the skin and subcutaneous cellular tissue of the face and head; but they may occur elsewhere. The popular belief is that they are caused by the longing of the mother during her pregnancy for a lobster, or strawberry or raspberry, or some other red-coloured article of food, and that the influence of her mind has impressed upon the fetus a more or less vivid image of the thing she longed for; and hence the name of mother-spot. Sometimes these tumours waste away spontaneously, and give no trouble; but frequently they increase rapidly, invade the adjacent tissues, and ulcerate or slough, and thus become dangerous to life by hæmorrhage. When these tumours do not show a tendency to increase no treatment is necessary, except to remove the disfigurement. When treatment is desirable many different methods may be employed, according to the form and situation of the tumour: e.g. removal by knife or ligature; coagulation of the contained blood by electrolysis; production of inflammation by application of caustics or, in infants, by vaccination upon the nāvus.