Dermatophyte
A dermatophyte is a fungus that digests keratin, allowing it to live in the outermost layer of the skin and in hair and nails. Dermatophytes cause the common superficial fungal infections grouped under the clinical prefix tinea.
A group of fungi defined by what they eat. Dermatophytes digest keratin, which means the outermost layer of skin, along with hair and nails, is food rather than an obstacle.
That diet explains their behaviour. They live where the keratin is, in tissue that is already dead, and in otherwise healthy skin they do not penetrate the living layers beneath. This is why the infections they cause are described as superficial, and why the redness and itch that accompany them are the immune system responding to their presence rather than damage they have inflicted at depth.
Growth spreads outwards from a starting point, so activity concentrates at the advancing edge while the centre quietens. That produces the ring with a raised rim and a clearer middle, and the name ringworm, which involves no worm and never did.
Clinically they are named by site rather than by species, using the prefix tinea with a Latin word: tinea corporis for the body, tinea pedis for the foot, tinea cruris for the groin, tinea capitis for the scalp, tinea unguium for the nail. That system makes an otherwise confusing set of common names suddenly orderly.
Yeasts, including Candida and the Malassezia group, cause separate superficial problems and are not dermatophytes, which is why they are discussed apart.
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