Nerve injuries are ranked by Seddon from least to most severe: neuropraxia < axonotmesis < neurotmesis. The question asks for the essence of the mildest grade.
Neuropraxia means the nerve simply stops conducting across a segment - a physiological block, usually due to focal demyelination or compression - while the axon itself and every surrounding connective-tissue layer stay structurally intact. Because no axon is cut, distal Wallerian degeneration does not occur, and once the demyelinated segment remyelinates, conduction returns fully, often within days to a few weeks.
$\text{Neuropraxia: conduction blocked,\ axon intact,\ no Wallerian degeneration}$
By contrast, “endoneurium cut” reflects axonotmesis/neurotmesis where the axon is interrupted, and “sheath cut” reflects neurotmesis with complete division requiring surgical repair. Therefore the correct description of neuropraxia is a reversible physiological conduction block.
\[\boxed{\text{Physiological (reversible) conduction block, axon intact}}\]