Final answer:
Beau's lines are horizontal indentations on the nail plates which indicate a temporary disruption in nail growth, often caused by systemic illness.
Step-by-step explanation:
Transverse depressions of the nail plates, usually bilateral, resulting from temporary disruption of proximal nail growth from systemic illness, is termed Beau's lines.
Beau's lines are indications of an underlying condition that has caused the nail matrix, a deep layer of epidermal tissue at the proximal end of a nail where nail growth occurs, to temporarily cease producing cells. This cessation can be due to various systemic illnesses, stress, or trauma. The nail plates are part of the nail accessory organ of the skin made of sheets of dead keratinocytes. When the nail matrix is disrupted, the nail plate's typically smooth surface reflects the disturbance as a horizontal groove or ridge.
This reflects the period when nail growth was interrupted. The health of the nail is a good indicator of overall systemic health, and Beau's lines are a classic sign of systemic upset that can be seen by examining the nail bed, the area of skin under the nail plate.