IMU

The Interactive Media Unit or IMU is a graphical advertising format for web pages.

It appears to be the new standard category for all internet advertising sizes set by the IAB, possibly making the term MPU out of date. Although MPU is generally used to refer to a particular ad size, often 300x250, the term IMU is applicable to all ad sizes. (http://www.iab.net/iab_products_and_industry_services/1421/1443/1452)