Part of the Hardware glossary: Unified Extensible Firmware Interface (UEFI) is a specification for a software program that connects a computer's firmware to its operating system (OS).UEFI is expected to eventually replace BIOS. Like BIOS, UEFI is installed at the time of manufacturing and is the first program that runs when a computer is turned on.he Unified Extensible Firmware Interface is managed by a group of chipset, hardware, system, firmware, and operating system vendors called the UEFI Forum.The specification is most often pronounced by naming the letters U-E-F-I.