Definition of moonmilk in US English:

moonmilk

noun

Geology
  • A soft whitish deposit of carbonate minerals found on the walls of limestone caves as a floury powder or porous mass; especially such a deposit consisting chiefly of calcite.

Origin

Late 19th century. From moon + milk, after German Mondmilch or post-classical Latin lac lunae (Gesner 1555).

Pronunciation

moonmilk

/ˈmuːnmɪlk/