Very great in size, extent, or quantity. See Synonyms at enormous. 2. Very great in scope or import: a vast improvement. n. Archaic An immense space. [Latin vāstus.] vast′ness n. American Heritage® Dictionary of the English Language, Fifth Edition.

from The American Heritage® Dictionary of the English Language, 5th Edition. adjective Very great in size, extent, or quantity. synonym: enormous. adjective Very great in scope or import. noun An immense space. from The Century Dictionary. Wide and vacant or unoccupied; waste; desolate; lonely.