Flux transition
The point on the medium where magnetisation reverses direction — the physical event that all magnetic storage fundamentally reads and counts.
A flux transition is the place along a track where the direction of the magnetic field flips from one polarity to the other. A read head senses not a static field but the change itself: each transition induces a voltage pulse. Every encoding on magnetic disk and tape — FM, MFM, RLL, GCR — is at bottom about how ones and zeros should be translated into a pattern of transitions and gaps that the head can read back unambiguously and without losing the clock.
The measure of how tightly transitions can be packed along a track is called linear density, given in flux reversals per inch. Together with track density it yields areal density. Pushing transitions closer together is exactly what write precompensation and channel techniques like PRML try to manage before neighbouring bits' fields start smearing into one another.