Q: What is Beau's lines? 

Answer: Beau's line appear as transverse lines across the nail as signs of previous acute illnesses like severe infection, mayocardial infarction, any kind of shock, hypocalcemia, any big surgery, immunosuppressive therapy or chemotherapy and severe zinc deficiency. 

Mote interestingly, location on the nail may even provide the approximate date of the illness and the depth of the line provides a clue to the severity of the illness!