Definitions of heavy metal: Survey of current usage (April 2001).
Definitions in terms of density (specific gravity)
- metals fall naturally into two groups–the light metals with densities below 4, and the heavy metals with densities above 7 
- metal having a specific gravity greater than 4 
- metal of high specific gravity, especially a metal having a specific gravity of 5.0 or greater 
- metal with a density greater than 5 
- metal with a density greater than 6 g/cm3 
- metal of specific gravity greater than 4 
- metal with a density of 5.0 or greater 
- metal whose specific gravity is approximately 5.0 or higher 
- metal with a density greater than 5 
- (in metallurgy) any metal or alloy of high specific gravity, especially one that has a density greater than 5 g/cm3 
- metal with a density higher than 4.5 g/cm3 
- metal with a density above 3.5-5 g/cm3 
- element with a density exceeding 6 g/cm3 
Definitions in terms of atomic weight (mass)
- metal with a high atomic weight 
- metal of atomic weight greater than sodium 
- metal of atomic weight greater than sodium (23) that forms soaps on reaction with fatty acids 
- metallic element with high atomic weight (e.g., mercury, chromium, cadmium, arsenic, and lead); can damage living things at low concentrations and tend to accumulate in the food chain 
metallic element with an atomic weight greater than 40 (starting with scandium; atomic number 21); excluded are alkaline earth metals, alkali metals, lanthanides, and actinides 
- metal with a high atomic mass 
heavy metals is a collective term for metals of high atomic mass, particularly those transition metals that are toxic and cannot be processed by living organisms, such as lead, mercury, and cadmium  metal such as mercury, lead, tin, and cadmium that has a relatively high atomic weight 
- rather vague term for any metal (in whatever chemical form) with a fairly high relative atomic mass, especially those that are significantly toxic (e.g., lead, cadmium, and mercury). They persist in the environment and can accumulate in plant and animal tissues. Mining and industrial wastes and sewage sludge are potential sources of heavy metal pollution .
- a metal such as cadmium, mercury, and lead that has a relatively high relative atomic mass. The term does not have a precise chemical meaning .
- metal with a high relative atomic mass. The term is usually applied to common transition metals such as copper, lead, or zinc .
Definitions in terms of atomic number
- in electron microscopy, metal of high atomic number used to introduce electron density into a biological specimen by staining, negative staining, or shadowing 
- in plant nutrition, a metal of moderate to high atomic number, e.g., Cu, Zn, Ni, or Pb, present in soils owing to an outcrop or mine spoil, preventing growth except for a few tolerant species and ecotypes 
- the rectangular block of elements in the Periodic Table flanked by titanium, hafnium, arsenic, and bismuth at its corners but including also selenium and tellurium. The specific gravities range from 4.5 to 22.5 .
- any metal with with an atomic number beyond that of calcium 
- any element with an atomic number greater than 20 
metal with an atomic number between 21 (scandium) and 92 (uranium) 
- term now often used to mean any metal with atomic number >20, but there is no general concurrence 
Definitions based on other chemical properties
- heavy metals is the name of a range of very dense alloys used for radiation screening or balancing purposes. Densities range from 14.5 for 76% W, 20% Cu, 4% Ni to 16.6 for 90% W, 7% Ni, 3% Cu .
- intermetallic compound of iron and tin (FeSn2) formed in tinning pots that have become badly contaminated with iron. The compound tends to settle to the bottom of the pot as solid crystals and can be removed with a perforated ladle .
- lead, zinc, and alkaline earth metals that react with fatty acids to form soaps. "Heavy metal soaps" are used in lubricating greases, paint dryers, and fungicides .
- any of the metals that react readily with dithizone (C6 H5 N), e.g., zinc, copper, lead, etc. .
- metallic elements of relatively high molecular weight .
Definitions without a clear basis other than toxicity
- element commonly used in industry and generically toxic to animals and to aerobic and anaerobic processes, but not every one is dense or entirely metallic; includes As, Cd, Cr, Cu, Pb, Hg, Ni, Se, and Zn 
- outdated generic term referring to lead, cadmium, mercury, and some other elements that generally are relatively toxic in nature; recently, the term "toxic elements" has been used. The term also sometimes refers to compounds containing these elements .
Definitions preceding 1936
- guns or shot of large size 
- great ability 
The term "heavy metal" has never been defined by any authoritative body such as IUPAC. Over the 60 years or so in which it has been used in chemistry, it has been given such a wide range of meanings by different authors that it is effectively meaningless. No relationship can be found between density (specific gravity) or any of the other physicochemical concepts that have been used to define heavy metals and the toxicity or ecotoxicity attributed to heavy metals.
Source: International Union of Pure and Applied Chemistry