Guard band
The unused magnetic space between two tracks, there for one reason only: to stop neighbouring tracks from leaking into each other.
When tracks sit too close, the head picks up field from the neighbour — crosstalk that drowns the weak wanted signal. The fix is to leave a narrow empty strip between tracks: a guard band. It costs area, and therefore capacity, so the whole story of rising track density is a war against the width of the guard band.
Shingled magnetic recording takes it all the way and abolishes guard bands by overlapping tracks like roof tiles — hence the capacity gain, and hence why rewriting becomes expensive since you disturb the neighbouring track. The term also appears in radio engineering for frequency bands, but that is a different thing entirely.