An SO-DIMM, or small outline dual in-line memory module, is a type of computer memory built using integrated circuits.
SO-DIMMs (also written SODIMMs) are a smaller alternative to a DIMM, being roughly half the size of regular DIMMs. SO-DIMMs are often used in systems which have space restrictions such as notebooks, small footprint PCs (such as those with a Mini-ITX motherboard), high-end upgradable office printers, and networking hardware like routers.
Read more about SO-DIMM: Visual Identification, General Characteristics