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What is Nevus and explain in detail?
Nevus (plural nevi) is a nonspecific medical term for a visible, circumscribed, chronic lesion of the skin or mucosa. The term originates from nævus, which is Latin for "birthmark"; however, a nevus can be either congenital (present at birth) or acquired. Common terms, including mole, birthmark, and beauty mark, are used to describe nevi, but these terms do not distinguish specific types of nevi from one another.

Classification
The term nevus is applied to a number of conditions caused by neoplasias and hyperplasias of melanocytes, as well as a number of pigmentation disorders, both hypermelanotic (containing increased melanin, the pigment responsible for skin color) and hypomelanotic (containing decreased melanin). Suspicious skin moles which are multi-colored or pink may be a finding in skin cancer.

Increased melanin
Usually acquired
Melanocytic nevus
Melanocytic nevi can be categorized based on the location of melanocytic cellsJunctional: epidermis
Intradermal: dermis
Compound: epidermis and dermis
Atypical (dysplastic) nevus: This type of nevus must be diagnosed based on histological features. Clinically, atypical nevi are characterized by variable pigmentation and irregular borders.
Beckers nevus
Blue nevus (rarely congenital): A classic blue nevus is usually smaller than 1 cm, flat, and blue-black in color.
Horis nevus
Nevus spilus (speckled lentiginous nevus): This lesion includes dark speckles within a tan-brown background.
Pigmented spindle cell nevus
Spitz nevus
Zosteriform lentiginous nevus

Usually congenital
Congenital melanocytic nevus
These nevi are often categorized based on size, however, the lesions usually grow in proportion to the body over time, so the category may change over an individuals life. This categorization is important because large congenital melanocytic nevi are associated with an increased risk of melanoma, a serious type of skin cancer.Small: <1.5 cm
Medium: 1.5–19.9 cm
Large: ≥ 20 cm
Nevus of Ito
Nevus of Ota

Decreased melanin
Acquired
Nevus anemicus

Congenital
Nevus depigmentosus
		
Additional types of nevi do not involve disorders of pigmentation or melanocytes. These additional nevi represent hamartomatous proliferations of the epithelium, connective tissue, and vascular malformations.

Epidermal nevi
These nevi represent excess growth of specific cells types found in the skin, including those that make up oil and sweat glands.
Verrucous epidermal nevus
Nevus sebaceous
Nevus comedonicus
Eccrine nevus
Apocrine nevus

Connective tissue nevi
Connective tissue nevi represent abnormalities of collagen in the dermis, the deep layer of the skin.
Collagenoma
Elastoma

Vascular nevi
These nevi represent excess growth of blood vessels, including capillaries.
Nevus simplex (nevus flammeus nuchae), also known as a stork bite or salmon patch.

Intramucosal nevi
An intramucosal nevus is a nevus within the mucosa as found in for example the mouth and genital areas. In the mouth, they are found most frequently on the hard palate. They are typically light brown and dome-shaped. Intramucosal nevi account for 64% of all reported case of oral nevi.

Diagnosis
Nevi are typically diagnosed clinically with the naked eye or using dermatoscopy