A very imprecise term, never defined by any authoritive body, used loosely to refer to both the element and its compounds. It is based on categorization by density, which is rarely a biologically significant property. It is often used as a group name for metals and metalloids that have been associated with contamination and potential toxicity or ecotoxicity. However, the assumption that all so-called "heavy metals" and their compounds have highly toxic or ecotoxic properties is not supported by facts. Even further, the list of "heavy metals" is not clearly defined and has no basis in their chemistry. Even the term "metal" is commonly misused in both toxicological literature and in legislation to mean the pure metal and all the chemical species in which it may exist.

Very often, ther term "heavy metals" is used, based on one of the following "definitions":

• In terms of density: Metals having a density greater than 4 (4.5, 5 or 6 g/cm3)
• In terms of atomic weight: Metals with a atomic weight greater than 23 (40)
• In terms of atomic number: Metals with an atomic number greater than 20

The use of the term "heavy metals"  is highly discorouged  by IUPAC.