Extensible Host Controller Interface
The eXtensible Host Controller Interface (xHCI) is a technical specification that provides a detailed framework for the functioning of a computer's host controller for Universal Serial Bus (USB). Known alternately as the USB 3.0 host controller specification, xHCI is designed to be backward compatible, supporting a wide range of USB devices from older USB 1.x to the more recent USB 3.x versions.
Source: Wikipedia — Extensible Host Controller Interface (CC BY-SA 4.0)