It stands for Wireless Interoperability for Microwave Access. It is an IP based wireless broadband technology that provides performance similar to Wi-Fi with coverage and QoS of cellular networks.
It uses licensed spectrum of 2.3Ghz, 2.5Ghz and 3.5Ghz as well as unlicensed spectrum of 3.65Ghz and 5.8Ghz.
There is no global standard as to which spectrum is to be used by WiMax. Although WiMax is commonly known as 4G technology yet it is not as dost fulfill the requirements laid out by ITU for 4G. To understand better what is 4G, please click here.