The Free On-line Dictionary of Computing (30 December 2018):
A storage device built using materials such as gadolinium
gallium garnet which are can be magnetised easily in only one
direction. A film of these materials can be created so that
it is magnetisable in an up-down direction. The magnetic
fields tend to join together, some with the north pole facing
up, some with the south.
When a veritcal magnetic field is imposed on this, the areas
in opposite alignment to the field shrink to circles, or
'bubbles'. A bubble can be formed by reversing the field in a
small spot, and can be destroyed by increasing the field.
Bubble memory is a kind of non-volatile storage but
EEPROM, Flash Erasable Programmable Read-Only Memory and
ferroelectric technologies, which are also non-volatile, are
["Great Microprocessors of the Past and Present", V 4.0.0,
John Bayko , Appendix C]