Dual Inline Memory Module
A 64-bit memory module used im many moderm computers beginning in about 1996. The most common variety is the 168-pin SDRAM type, though DIMMS for EDO and DDR SDRAM also are used. There is also a small form-factor version, SO-DIMM or Small Outline DIMM, for use in mobile and compact electronics.
But Where does the 'Dual' come from?
There are two schools of thought on this, either could be right. The first comes from recent microcomputer systems having 64-bit data pathways, you would need two 72-pin SIMM modules installed, because a SIMM is 32-bit and most current CPUs, while 32-bit in design, have dual data pipelines, and 32-bits are needed for each. Hence, a DIMM being 64-bit, you only needed one DIMM to power a Pentium and so it did the job of two SIMMs.
or, more likely...
On 30- and 72-pin SIMMs, the edge-connector pins on either side of the circuit board that comprises the SIMM are wired together, so pin 23 on the front is the same as 23 on the back. But on a 168-pin DIMM, the forward and leaward pins of the same number have different signals -- so they are dual sided verses single sided. There's the DUAL. | 
