A digital HD video and audio interface for connecting mobile phones and portable devices to HDTVs and other home entertainment products. It piggybacks on the standard micro-USB connector.
MHL was only ever offered on a few high-end phones before USB-C started to replace micro-USB, starting with high-end phones. USB-C offers alternate (all-digital) ways to replicate the functionality of MHL, making MHL obsolete.
By using an existing connector on the phone, MHL eliminates the need for a separate (micro) HDMI connector. This allows MHL phones to be smaller and lighter while still providing wired TV-output functionality.
A typical MHL cable will have a special micro-USB connector on one end, for connecting to the phone, and a standard HDMI connector on the other end, for connecting to a TV, etc. The cable usually must be purchased separately.
MHL connectors are actually modified versions of micro-USB, with a slightly longer male connector and deeper female connector, to accommodate extra pins. An extra bump on the male connector ensures it will not fit easily into an incompatible (non-MHL) phone. The female connector (in the phone) remains compatible with standard micro-USB plugs for charging, data transfer, etc.
MHL supports 1080p HD video and digital audio. It also simultaneously provides power to the mobile device. It can also enable the TV remote to control the mobile phone and its media content.
A competing standard that also connects a phone's USB port to a TV or monitor is SlimPort.
The MHL Consortium was originally comprised of Nokia, Samsung Electronics, Silicon Image, Sony, and Toshiba.