Integrated Facility for Linux

The Integrated Facility for Linux (IFL) is an IBM mainframe and Power Systems processor dedicated to running the Linux operating system. On IBM Z and IBM LinuxONE machines, IFLs can be used with or without hypervisors such as z/VM and KVM. IFLs are one of three most common types of "specialty" IBM mainframe processors that give software vendors (and their customers) more granular control over software licensing and maintenance costs.

Source: Wikipedia — Integrated Facility for Linux (CC BY-SA 4.0)