Definition of "constituent"

Body, soul, and reason are the three parts necessarily constituent of a man.

1695, C[harles] A[lphonse] du Fresnoy, translated by John Dryden, De Arte Graphica. The Art of Painting, […], London: […] J[ohn] Heptinstall for W. Rogers, […]

A question of right arises between the constituent and representative body.

1769, Junius, letter on 19 December, 1769, (part of Letters of Junius)

We know how to bring these constituents together, and to cause them to form water.

1865, John Tyndall, The Constitution of the Universe, published 1869, page 11

whose first composure and origination requires a higher and nobler Constituent than either Chance or the ordinary method of meer Natural causes.

a. 1677, Matthew Hale, The Primitive Origination of Mankind, Considered and Examined According to the Light of Nature, London: […] William Godbid, for William Shrowsbery, […], published 1677

To appeal from the representatives to the constituents.

1849–1861, Thomas Babington Macaulay, chapter 25, in The History of England from the Accession of James the Second, volumes (please specify |volume=I to V), London: Longman, Brown, Green, and Longmans

But the purported rise in violent videos online has led some MPs to campaign for courts to have more power to remove or block material on YouTube. The Labour MP Heidi Alexander said she was appalled after a constituent was robbed at knifepoint, and the attackers could be found brandishing weapons and rapping about gang violence online.

2012 April 19, Josh Halliday, “Free speech haven or lawless cesspool – can the internet be civilised?”, in the Guardian‎

Thus, the postulation of a Noun Phrase constituent is justified on morphological grounds, since it is not obvious how we could describe the grammar of the genitive s inflection in English without saying that it's a Noun Phrase inflection.

1988, Andrew Radford, Transformational grammar: a first course, Cambridge, UK: Cambridge University Press, page 65