Stands for "Single In-Line Memory Module."
A SIMM is a small circuit board that holds memory chips, and is also known as a RAM stick. A SIMM is installed into a slot on a computer motherboard, allowing the RAM configuration in a computer to be customized. The amount of RAM a SIMM contains depends on the number of memory chips on it, as well as the capacity of each of those chips.
Before the introduction of SIMMs, computer memory was installed one chip at a time into sockets on a motherboard. SIMMs were developed to make it easier to install a larger amount of memory at once, and to save space on the motherboard. Early SIMMs had 30 contact pins, and supported an 8-bit data bus. Later SIMMs increased the pin count to 72, added an off-center notch to guide installation, and utilized a 32-bit data bus to the CPU. Once computers began using a 64-bit bus, SIMMs had to be installed in matching pairs in order to work properly.
SIMMs were eventually replaced as the standard memory module by DIMMs, which supported a 64-bit data bus. While a SIMM has one set of electrical contact pins that are identical on both sides of the module, each side of a DIMM has separate pins. DIMMs could also be used one at a time, instead of requiring matching pairs.