Heavy ICBM is a term that was created in the 1970s to describe a class of SovietICBMs (intercontinental ballistic missiles). They were characterized by a heavy throw-weight of 5 to 9 metric tons (hence the word, "heavy," in the name) and a length of over 35 meters, and were thus capable of delivering a large number of warheads in a single MIRV missile.
This term usually refers only to SS-9 and SS-18 missiles. In time these missiles became quite feared in the United States because of their ability to destroy almost all U.S. silo-based ICBMs in a single strike, and because the U.S. had no comparable weapon. It was the foundation of the U.S. "second missile gap," which arose in the early 1980s and was one of the main reasons for Ronald Reagan's strategic armament buildup.