A drop in the nervous system's ability to drive muscle, which shows as weakness and poor coordination on days when the muscles themselves feel fine.
The term gets used loosely in gyms, usually to explain any bad session. The real phenomenon is narrower: measurable central fatigue follows very heavy, very long or very novel work, and it resolves in days rather than weeks.
What a trainer can actually observe is the pattern. Bar speed down at weights that normally move well, technique degrading earlier than usual, RPE up two points at the same load, and it showing on every lift rather than one. Local muscle fatigue is specific to a lift; this is not.
The response is the same either way: cut volume for a week, keep some intensity so nothing detrains, and check sleep before changing the program. If two deloads in a row do not clear it, the problem is outside training.