a DIMM
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Picture buying milk: rather than carry single cartons home one at a time, you grab a whole crate that bundles many cartons onto one easy-to-handle tray. A DIMM is the crate for memory chips: a small printed-circuit stick that groups a row of individual DRAM chips onto one module you snap into a slot on the motherboard. You almost never buy a bare DRAM chip — you buy DIMMs, because that is the practical packaging.
Concretely, DIMM stands for dual in-line memory module: a circuit board with DRAM chips on it and a row of gold contacts along the bottom edge that plug into a memory slot. The chips on a DIMM are wired so that together they present a wide data path — a standard DIMM forms a 64-bit-wide data interface (72 bits with ECC), spread across several chips that are read in parallel. DIMMs may have chips on one or both sides and may be organized into ranks (groups of chips activated together), and they come in flavours like UDIMM (unbuffered), RDIMM (registered/buffered, for servers), and SO-DIMM (small, for laptops).
Why it matters: the DIMM is the unit you actually handle, buy, and install, and it is what plugs into a memory channel. How many DIMMs and how they map onto channels directly shapes capacity and bandwidth. An honest caution: more DIMMs is not automatically more bandwidth — bandwidth comes from channels working in parallel, so two DIMMs on two separate channels can outrun two DIMMs crowded onto one channel. Matching DIMMs to channels (and populating channels evenly) is how you actually unlock memory performance.
Installing two 8 GiB DIMMs gives 16 GiB total either way — but placing them in two different channels enables dual-channel operation and roughly doubles peak bandwidth versus stacking both on one channel.
Capacity comes from total DIMM size; bandwidth comes from how DIMMs map onto channels.
A DIMM is just packaging — it does not change DRAM's fundamental latency. And RDIMMs, UDIMMs, and ECC vs non-ECC are generally not interchangeable; the motherboard and CPU dictate which type you may use.