The Free On-line Dictionary of Computing (30 December 2018):
ferrite core memory
(Or "core") An early form of non-volatile storage
built (by hand) from tiny rings of magnetisable material
threaded onto very fine wire to form large (e.g. 13"x13" or
more) rectangluar arrays. Each core stored one bit of data.
These were sandwiched between printed circuit boards(?).
Sets of wires ran horizontally and vertically and where a
vertical and horizontal wire crossed, a core had both wires
threaded through it.
A single core could be selected and magnetised by passing
sufficient current through its horizontal and vertical wires.
A core would retain its magnetisation until it was
re-magnetised. The two possible polarities of magnetisation
were used to represent the binary values zero and one.
A third "sense" wire, passed through the core and, if the
magnetisation of the core was changed, a small pulse would be
induced in the sense wire which could be detected and used to
deduce the core's original state.
Some core memory was immersed in a bath of heated oil to
improve its performance.
Core memory was rendered obsolete by semiconductor memory.
For example, the 1970s-era NCR 499 had two boards, each with
16 kilobytes of core memory.