What condition is characterized by horizontal lines across the nails?

Beau's lines are transverse grooves that run horizontally across the nails and are indicative of a temporary interruption in nail growth due to various physiological stressors. These lines can appear as a result of a number of conditions, including severe illness, malnutrition, or a systemic condition that affects body metabolism. The presence of Beau's lines signifies that the nail matrix experienced some form of trauma or illness, causing the nail to stop growing for a while, leading to these distinct lines.

In contrast, the other options are associated with different nail or skin conditions. The lunula refers to the crescent-shaped area at the base of the nail and is not a line but rather an anatomical feature of the nail. Onycholysis involves the separation of the nail from the nail bed, which does not specifically produce horizontal lines. A cyst is a sac-like structure that can occur under the skin or in other body tissues but is unrelated to the appearance of horizontal lines on the nails. Therefore, the clear identification of Beau's lines as the correct answer highlights their role as a diagnostic indicator for underlying health issues.