A wireless LAN (WLAN) specification under development by the IEEE (Institute of Electrical and Electronics Engineers) that delivers wireless data transfer rates in the range of 433 Mbps (Megabits per second) per spatial stream. With support for up to eight streams, the 802.11ac specification offers a theoretical maximum data transfer speed of more than 3Gbps (Gigabits per second), and can deliver 1.3Gbps transfer speeds with a more common three-antenna (three streams) design.
Recommended Reading: See 802.11 for a complete overview of the 802.11x family of specifications.
802.11ac extends earlier 802.11 specifications, including 802.11n, and provides backwards compatibility with previous 802.11 specs. The 802.11ac specification operates only in the 5 GHz frequency range (where there is currently far less potential for interference), as opposed to the 802.11n spec, which operates in both 5 GHz and 2.4 GHz ranges.
The official working name for the 802.11ac spec is Enhancements for Very High Throughput for WLAN operation in bands below 6 GHz, and is more informally referred to as Gigabit Wi-Fi or 5G Wi-Fi. The 802.11ac specification has already received an update in the form of 802.11ac Wave 2, which utilizes MU-MIMO technology and other advancements to help increase theoretical maximum wireless speeds for the spec to 6.93 Gbps.
Also see Webopedia’s Wireless LAN Standards chart and How Wireless Networks Work.