What is meant by mass defect of a single neutron or a single proton?
The reduction in its mass.
The mass defect of a nucleus represents the mass of the energy binding the nucleus, and is the difference between the mass of a nucleus and the sum of the masses of the nucleons of which it is composed.
It isn't quite that, in that binding energy is negative, and there isn't any actual thing comprised of negative energy. There's less positive energy present, that's all, because the mass of each nucleon is reduced.
Then, what is meant by the mass defect of a single neutron or proton?
A reduced mass. In physics people often say rest mass is invariant mass. That's misleading, because mass varies. See the Wikipedia binding energy article along with mass in general relativity:
"This missing mass may be lost during the process of binding as energy in the form of heat or light, with the removed energy corresponding to removed mass through Einstein's equation E = mc²."
"In special relativity, the invariant mass of a single particle is always Lorentz invariant. Can the same thing be said for the mass of a system of particles in general relativity? Surprisingly, the answer is no. A system must either be isolated, or have zero volume, in order for its mass to be Lorentz invariant. While the density of energy momentum, the stress–energy tensor is always Lorentz covariant, the same cannot be said for the total energy–momentum. (Nakamura, 2005). Non-covariance of the energy–momentum four-vector implies non-invariance of its length, the invariant mass."
When you lift a brick you do work on it. You add energy to it. You increase its mass. When you drop it, this extra mass-energy, which we call potential energy, is converted into kinetic energy. When the brick hits the ground this kinetic is radiated away, and you're left with a mass deficit. The Earth is also effected but not as much. Momentum is equal and opposite but kinetic energy isn't. So we tend to ignore the Earth. It's somewhat similar for the electron and the proton which combine to form a hydrogen atom. The mass of this is 13.6ev/c² less than that of the free electron and the free proton. The electron has lost most of the mass. The binding energy is a bit more dramatic in a nucleus, see hyperphysics.