Single Inline Memory Module
From Computer History Wiki
A Single Inline Memory Module (usually abbreviated to SIMM) was a standard main memory technology in the early Pentium era.
These memory chips came on a small printed circuit board that would snap in place in a socket on the machine's motherboard. These were far easier to install, remove and upgrade than the old sets of DIP chips.
SIMMs were typically known for how many pins they had, with the common types being 30 pin & 72pin SIMMs.