A single quark, sitting all by itself. Actually, free quarks do not exist in nature. The strong nuclear force, for some reason, binds together particles so that their "color sum" is zero. Thus we either see trios of red, green, and blue quarks, or system of quarks and anti-quarks. The trios are protons and neutrons, and the combinations involving antimatter are mesons.
To tear away a single quark from such a combination would require infinite energy. At high energy levels (as in large particle accelerators), however, quarks behave almost as if they were free and can be observed individually. Also, the energy expended in trying to separate them can materialise in showers of particles, which can also be observed. Thus the concept of individual quarks is well defined, even though we never see them.