Stands for "Small Outline Dual In-line Memory Module."
A SO-DIMM is a compact memory module roughly half the size of a full-size DIMM module. Like a DIMM, a SO-DIM is a small circuit board containing memory chips that serve as a computer's RAM. Laptop computers and some compact desktops use SO-DIMM modules, as do upgradeable office printers, routers, and NAS devices. Despite their small size, SO-DIMMs are generally available in similar capacities as full-size DIMMs and typically only cost slightly more due to the extra cost of miniaturizing components.
Since laptop computers are designed to be compact, they don't have the physical space necessary for full-sized DIMM modules. Instead, they use compact SO-DIMM modules designed to lay flat against the laptop's motherboard (instead of sticking up perpendicular to the motherboard, as in desktop computers). Some laptops make their RAM user-accessible by providing a small door or access panel, allowing you to replace old modules with new, higher-capacity ones.
Like full-size DIMMs, each new generation of SO-DIMMs changes the pin count and notch position to prevent incompatible memory from being installed. The earliest SO-DIMM modules only used 72 pins to connect to the laptop's motherboard, but DDR SO-DIMMs use nearly as many pins as their full-size counterparts — DDR1 and DDR2 modules use 200 pins, DDR3 modules use 204, DDR4 uses 260, and DDR5 SO-DIMM modules use 262 pins.