Keratoid is a term used to refer to pathological conditions characterized by the formation of a hard, horn-like lesion. Some synonyms for this term include keratinous, cornified, and sclerotic. Keratinous refers to the formation of keratin, the tough, fibrous protein that makes up the outer layer of the skin, nails, and hair. Cornified refers to the hardening of the skin due to the accumulation of dead skin cells. Sclerotic refers to the hardening and thickening of tissues due to the deposition of collagen fibers. These terms are often used interchangeably to describe conditions such as keratosis, corns, calluses, and sclerosing tumors.