Stands for "Single In-Line Memory Module." This is an older type of computer memory. A SIMM is a small circuit board with a bunch of memory chips on it. SIMMs use a 32-bit bus, which is not as wide as the 64-bit bus dual in-line memory modules (DIMMs) use. Newer processors require a 64-bit memory bus, so it is best to use DIMMs. Sometimes you can get away with installing SIMMS, but they have to be installed in pairs.