Devicetree
A text file describing what hardware a board has, so the same kernel can run on many boards without being rebuilt.
On a PC the operating system discovers hardware by itself through PCI and ACPI. On an embedded board, UARTs, I²C controllers and GPIO sit at fixed addresses that nothing can be asked about. For years Linux handled this with one C file per board, and in 2011 Linus Torvalds publicly lost patience with the pile. ARM then moved to devicetree, which PowerPC already used and which traces back to Sun's Open Firmware.
The source (.dts) is compiled by dtc into a binary blob (.dtb) that the bootloader, often U-Boot, hands to the kernel at boot. Zephyr uses the same format at build time instead: the tree becomes C macros, and no blob survives into the finished firmware.