Unified Extensible Firmware Interface - Wikipedia
https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface
The Unified Extensible Firmware Interface (UEFI) is a specification that defines a software interface between an operating system and platform firmware.
UEFI FAQs | Unified Extensible Firmware Interface Forum
https://uefi.org/faq
UEFI stands for "Unified Extensible Firmware Interface." The UEFI Specification defines a new model for the interface between personal-computer operating systems and platform firmware.
UEFI - OSDev Wiki
https://wiki.osdev.org/UEFI
(U)EFI or (Unified) Extensible Firmware Interface is a specification for x86, x86-64, ARM, and Itanium platforms that defines a software interface between the operating system and the platform firmware/BIOS.