IEEE 802.11n introduced the Wi-Fi generation known as Wi-Fi 4.
Published in 2009, 802.11n significantly improved wireless networking by using multiple-input multiple-output, or MIMO, technology. MIMO uses multiple antennas to transmit and receive several spatial streams, increasing throughput and helping performance in challenging radio environments.
The standard could operate in both the 2.4 GHz and 5 GHz radio bands. That dual-band capability helped it fit a wide range of routers and devices, although actual performance depended on channel width, antenna configuration, interference, and client support.
The Wi-Fi Alliance later applied generation numbers to make product names easier to understand. Under that naming system, 802.11n became Wi-Fi 4, 802.11ac became Wi-Fi 5, and 802.11ax became Wi-Fi 6. These labels identify generations, not guaranteed speeds.