Head gap
The tiny non-magnetic slit in the ring-shaped head where the field leaks out to magnetise the medium.
A classic write/read head is a ring of magnetic material with a narrow gap of non-magnetic material. When current flows in the winding the field is forced to bulge out across the gap, and it's that fringing field that writes the medium. The gap length sets a floor on wavelength: too long and adjacent transitions blur together, and on read the gap sets the resolution.
Ferrite and later thin-film heads shrank the gap to a fraction of a micron. When magnetoresistive heads arrived the write and read elements could be split apart, each with a gap tuned for its job — writing strongly versus reading finely.