Stands for "Synchronous Dynamic Random Access Memory."
SDRAM is a type of computer memory that synchronizes its data transfer actions with the system clock. Since it operates on the same clock cycles as the computer's CPU, it transfers data to and from the processor more efficiently and with less latency than older asynchronous memory. SDRAM data transfer rates are measured in MHz and represent how many times per second it can read from or write to its memory cells.
Earlier types of DRAM were asynchronous, which meant that the CPU would request data from the memory controller, and the memory would respond on its own time. Since the two did not share the same system clock, one component would either send a message that the other wasn't ready to receive or have to wait for a delayed reply. By synchronizing the CPU and the RAM to the same system clock cycle, both components knew exactly when to transfer data back and forth. The memory knows when to listen for a request from the CPU, and the CPU knows when the requested data will be available, so neither component is stuck waiting for the other to be ready.
SDRAM modules also store data in several equally-sized memory banks and access those banks in an interleaved manner. Alternating between banks allows an SDRAM module to work faster, as the memory controller can make a request to one bank while waiting for another to fulfill the previous request.
Nearly all computer system memory used since 2000 has been some form of SDRAM. The first generation, known as Single Data Rate (SDR) SDRAM, could perform a read or write action once per clock cycle. Subsequent generations of SDRAM can perform two actions per clock cycle, once on the rising edge of the electrical signal and once on the falling edge, and are known as Double Data Rate (DDR) SDRAM. As of 2023, DDR5 is the fastest available type of SDRAM.