A dummy word is a grammatical unit that has no meaning, but completes a sentence to make it grammatical.
It is raining.
Does he know?
Pei, Mario A., and Frank Gaynor. 1954.A dictionary of linguistics. New York: Philosophical Library.
Mish, Frederick (editor). 1991.Webster's ninth new collegiate dictionary. Springfield, MA: Merriam-Webster. 1,564 pages. 0877795088; indexed 0877795096; deluxe 087779510X.
Hartmann, R.R.K., and F.C. Stork. 1972.Dictionary of language and linguistics. London: Applied Science.
Crystal, David. 1985.A dictionary of linguistics and phonetics. 2nd edition. New York: Basil Blackwell.