A constituent quark is a current quark

Current quark

Current quarks are defined as the constituent quark cores of a valence quark....

 with a covering.

In the low energy limit of QCD

Quantum chromodynamics

In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...

, a description by means of perturbation theory is not possible. Here, no Asymptotic freedom

Asymptotic freedom

In physics, asymptotic freedom is a property of some gauge theories that causes interactions between particles to become arbitrarily weak at energy scales that become arbitrarily large, or, equivalently, at length scales that become arbitrarily small .Asymptotic freedom is a feature of quantum...

 exists, but the interactions between valence quarks and sea quarks gain strongly on significance. Part of the effects of virtual quarks and virtual gluons in the 'sea' can be assigned to a quark

Quark

A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...

 so well that the term 'constituent quark' seems appropriate.

According to the Feynman diagram

Feynman diagram

Feynman diagrams are a pictorial representation scheme for the mathematical expressions governing the behavior of subatomic particles, first developed by the Nobel Prize-winning American physicist Richard Feynman, and first introduced in 1948...

s, constituent quarks seem to be 'dressed' current quarks, i.e. current quarks surrounded by a cloud of virtual quarks and gluons. This cloud in the end explains the large constituent-quark masses.

Definition: Constituent quarks are valence quarks for which the correlations for the description of hadrons by means of gluons and sea-quarks are put into effective quark masses of these valence quarks.

The effective quark mass is called constituent quark mass. Hadrons consist of 'glued' constituent quarks. The use of positions for the light constituent quarks is not exactly unproblematic in the description of hadrons.

Binding energy

The quantum chromodynamic binding energy of a valence quark in a hadron is the amount of energy required to make the hadron spontaneously emit a meson containing the valence quark. This is the same as the constituent quark mass.

Constituent quark	Mass	Δx
Up quark Up quark The up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, forms the neutrons and protons of atomic nuclei...
Down quark Down quark The down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
Strange quark Strange quark The strange quark or s quark is the third-lightest of all quarks, a type of elementary particle. Strange quarks are found in hadrons, which are subatomic particles. Example of hadrons containing strange quarks include kaons , strange D mesons , Sigma baryons , and other strange particles...
Charm quark Charm quark The charm quark or c quark is the third most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks...
Bottom quark Bottom quark The bottom quark, also known as the beauty quark, is a third-generation quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large bare mass , combined with low values of the CKM matrix elements Vub and Vcb, gives it a...
Top quark Top quark The top quark, also known as the t quark or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin-, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and...

Describing hadrons using non-relativistic quantum mechanics becomes difficult.