Beau's lines: Transverse lines or grooves across the fingernails; transverse depressions in the nail plate. Beau's lines are caused by temporary cessation of cell division in the proximal nail matrix. The condition may be caused by local disease of the nail fold, physical trauma to it, or a systemic insult, such as an illness or a drug as, for example, chemotherapy. Named for the distinguished French physician Joseph Honore Simon Beau (1806-1865) who described this phenomenon in 1846.

Also known as Beau-Reil cross furrows. Johann Christian Reil (1759-1813), German anatomist, physiologist, and physician for the poet Johann Wolfgang von Goethe, described the same lines across the nails in 1796, a half century before Beau.