IMT-International Mobile Telecommunications

“IMT” or “International Mobile Telecommunications” is the umbrella term for mobile systems and services. It includes both 3G and 4G. There are various options and intermediate points in transitioning from 2G to 3G and 4G that are labelled 2.5G, 3.5G, 3.9G, etc. These “fractional” Gs are not formally recognized in standards but form part of the business positioning and marketing strategies of operators and manufacturers.

“IMT-2000” is the term used for third generation (3G) systems aimed at using the nominal 2 GHz frequency band with standards developed and products deployed in approximately year 2000, and providing smooth evolution paths to 3G from the various widely deployed existing 2G mobile networks.

“IMT Advanced” is the evolution of the terminology to reflect the addition of new radio technologies and advanced network infrastructure capabilities for the fourth generation (4G). 

Credit : http://www.itu.int/ITU-D/tech/MobileCommunications/IMT_INTRODUCING/IMT_Introducing.html