RRAM (resistive RAM)
Beginner
A memory cell that stores a number as how easily it lets electricity through. It keeps its value with the power off.
Novice
Resistive random-access memory: a thin oxide between two electrodes whose resistance can be switched, and often tuned to many levels, by voltage pulses. Non-volatile and small, so it can hold weights densely.
Expert
Conductive-filament devices; analog programming needs iterative write-verify. Device-to-device and cycle-to-cycle variation, conductance relaxation after programming, limited endurance and higher forming/programming voltages are the practical issues.
Explained in Sparsity, in-memory and analog compute (Architectures).
See also: Phase-change memory (PCM), Crossbar array (for computing).