Wikipedia
nvSRAM is a type of non-volatile random-access memory (NVRAM). It is similar in operation to static random-access memory (SRAM). The current market for non-volatile memory is dominated by BBSRAMs, or battery-backed static random-access memory. However, BBSRAMs are slow and suffer from RoHS compliance issues. nvSRAMs provide 20ns or lesser access times.
nvSRAM is one of the advanced NVRAM technologies that is fast replacing the BBSRAMs, especially for applications that need battery free solutions and long term retention at SRAM speeds. nvSRAMs are used in a wide range of situations—networking, aerospace, and medical, among many others —where the preservation of data is critical and where batteries are impractical.
nvSRAM is available from 16k densities up to 8M densities from both Simtek Corporation and Cypress Semiconductor. There are other nvSRAM products from Maxim which are essentially BBSRAMs. They have a lithium battery built into the SRAM package. It is an efficient replacement for BBSRAM, EPROM or EEPROM. It is faster than EPROM and EEPROM solutions. It is better than BBSRAM solution because there is no ROHS issue associated with this type of memory. No external battery is used.