What is the term for the thickening of the nail?

The term that refers to the thickening of the nail is "Onychauxis." This condition can occur for several reasons, including trauma, infection, or certain systemic diseases. When nails thicken, they can become more difficult to manage and may require treatment to address the underlying cause and improve their appearance.

Other terms presented in the choices serve different meanings; "Onyx" is a type of mineral and does not pertain to nail conditions. "Onychophagy" refers to the habit of nail-biting, which does not involve thickening but rather the opposite effect. Lastly, "Agnails," also known as hangnails, are small, torn pieces of skin near the nail, unrelated to the thickness of the nail itself.

Thus, "Onychauxis" is the precise term that identifies the condition of nail thickening.