The document discusses the memristor, which is the fourth fundamental circuit element along with the resistor, capacitor, and inductor. A memristor is a two-terminal device whose resistance depends on the amount of electric charge that has passed through it. This allows it to remember the value of electric current or voltage over time. Memristors have potential applications for non-volatile memory and neuromorphic computing due to their ability to mimic synaptic behavior in neurons. However, challenges remain in developing memristor technology, including a lack of design standards and issues with heat dissipation during writing and reading.