What does "foliation" mean?

Additional senses

1. 2.The act of beating a metal into a thin plate, leaf, foil, or lamina.
2. 3.The act of coating with an amalgam of tin foil and quicksilver, as in making looking-glasses.
3. 4.(Arch.) The enrichment of an opening by means of foils, arranged in trefoils, quatrefoils, etc.; also, one of the ornaments. See Tracery.
4. 5.(Geol.) The property, possessed by some crystalline rocks, of dividing into plates or slabs, which is due to the cleavage structure of one of the constituents, as mica or hornblende. It may sometimes include slaty structure or cleavage, though the latter is usually independent of any mineral constituent, and transverse to the bedding, it having been produced by pressure.