What is "salt rheum"

Eczema \Ec"ze*ma\, n. [NL., fr. Gr. 'e`kzema; "ek out + zei^n to boil.] (Med.) An inflammatory disease of the skin, characterized by the presence of redness and itching, an eruption of small vesicles, and the discharge of a watery exudation, which often dries up, leaving the skin covered with crusts; -- called also tetter, milk crust, and salt rheum.

n. (context medicine dated chiefly US English) Any of various cutaneous eruptions, especially eczema.