Bus mastering
When a device is allowed to start transfers on the bus itself rather than just answering when the CPU asks.
A bus has initiators and responders. For a long time the CPU was the only initiator. ISA cards in the PC made do with Intel's 8237, a central DMA controller that moved data on their behalf — third-party DMA, with cycle stealing as the price. PCI made bus mastering standard in 1992: the network card or disk controller could request the bus and write straight into RAM. In Linux the call is still pci_set_master().
In a microcontroller this is routine. The DMA controller, the Ethernet MAC and USB are all masters on AHB, and a bus matrix decides who goes first. The downside is security: a device that can write anywhere in memory can also read keys from it. Hence DMA attacks over FireWire and Thunderbolt, and hence the IOMMU.