The term 'Heavy Substance' can be described by the crossword solution of LEAD, which is a four-letter word. LEAD is a chemical element with the symbol Pb and atomic number 82. It is a heavy metal that is denser than most common materials. LEAD has been used since ancient times for a variety of purposes, including plumbing, roofing, and ammunition. It is also used in the production of batteries, pigments, and radiation shielding. LEAD is toxic to humans and can cause health problems if ingested or inhaled.