DMA Direct Memory Access
Peripherals reading and writing system memory on their own, without the processor copying every byte. Without DMA a modern disk or network card would needlessly occupy an entire core.
The processor sets up the transfer — address, length, direction — and is then interrupted only when it completes. That's the prerequisite for high throughput and for zero-copy, where data goes from network card to application memory with no intermediate copies. Security-wise it's a problem: a device allowed to write anywhere in RAM can read encryption keys or modify kernel code, which is exactly what DMA attacks over PCIe, Thunderbolt and FireWire have done. The countermeasure is an IOMMU, which gives each device its own restricted view of addresses — the same mechanism that lets a virtual machine get direct access to a graphics card.