What is the medical or technical term used to refer to nails?

The medical or technical term referring to nails is 'onyx.' This term derives from the Greek word for fingernail and is commonly used in dermatological and medical contexts to describe the hard surface of the nails. In nails, the primary component is keratin, a type of protein that forms the structure. While keratin is significant in understanding the composition of nails, the term specifically used to refer to nails themselves is onyx.

Other terms presented in the options have different meanings. 'Onycha' frequently refers to a historical resin or substance, often associated with perfumes and incense. 'Keratin' pertains to the protein structure forming the nails, skin, and hair but does not directly identify nails themselves. 'Manicure' is the beauty treatment focused on the care and aesthetic enhancement of nails, but it does not serve as a term for the nails themselves. Therefore, 'onyx' is the most accurate and precise term in this context.