A constituent is one of two or more grammatical units that enter syntactically or morphologically into a construction at any level.
The sentence You eat bananas contains the following constituents:
Quirk, Randolph, S. Greenbaum, G. Leech, and J. Svartvik. 1985.A comprehensive grammar of the English language. London: Longman.
Mish, Frederick (editor). 1991.Webster's ninth new collegiate dictionary. Springfield, MA: Merriam-Webster. 1,564 pages. 0877795088; indexed 0877795096; deluxe 087779510X.
Crystal, David. 1985.A dictionary of linguistics and phonetics. 2nd edition. New York: Basil Blackwell.