A SIMM (short for single in-line memory module) is a memory module consisting of six to nine RAM chips on a circuit board. There are two types of SIMMs - 30-pin modules, used in x86 machines, and 72-pin modules, used in 486 and Pentium systems. The memory chips on a SIMM module are usually DRAM or SDRAM chips.
See also DIMM (Dual In-Line Memory Module).