MiFi is a portable broadband device that allows multiple end users and mobile devices to share a 3G or 4G mobile broadband Internet connection and create an ad-hoc network.
Although MiFis are manufactured by Novatel, the name is often used as a generic label for any device capable of becoming a personal hotspot or pocket router. Although some smartphones have the capability to be used as hotspots, they do not have the management capabilities that Mifi offers to control data consumption on connected devices. Additionally, because the phone is not a dedicated device like Mifi, the phone's owner must turn the hotspot feature on whenever sharing is desired -- and during that time, the phone itself cannot behave as a Wi-Fi client. On some phones, taking a voice call disrupts 3G/4G data flow, suspending personal mobile hotspot service for other Wi-Fi devices using it at that time.
The name MiFi is thought to stand for "my WiFi." Some 3G/4G carriers sell MiFis (or similar devices) with pay-as-you-go plans, while others require a monthly fee or annual contract.
See also: tethering