What do you call the deep grooves or horizontal lines on nails?

The deep grooves or horizontal lines on nails are known as Beau's lines. These lines can develop due to a variety of factors, including trauma, systemic illness, or nutritional deficiencies. They are a manifestation of temporary interruption in nail growth that occurs during periods of stress on the body, such as severe illness or surgery.

In contrast to this, onycholysis refers to the separation of the nail from the nail bed, which is different in appearance and underlying cause. Choledocholithiasis pertains to the presence of gallstones in the common bile duct and is unrelated to nail conditions. A cyst is a fluid-filled sac and does not describe any specific condition related to nail grooves. Therefore, Beau's lines is the appropriate term to describe the specific phenomenon of deep grooves or horizontal lines present on the nails.