What constitutes a heavy metal varies based on context; the term can apply to both density and atomic weight, for instance. In common usage, however, the term typically refers to metals that can accumulate in the environment and cause harm to plant and animal tissues—e.g., lead, cadmium, and mercury. While these elements are naturally occurring, human activities have concentrated them in many areas, and they can remain in the soil for thousands or tens of thousands of years. Weisman uses this as an example of the ways in which humanity would continue to impact the environment even after its extinction.

Megafauna are animals large enough to be seen with the naked eye. Outside of technical scientific literature, however, this term is most commonly used to describe very large animals (i.e., comparable to or larger than humans) such as bears, lions, or elephants, which are often referred to as “charismatic megafauna.” The latter are Weisman’s primary concern in The World Without Us, as when he discusses the Pleistocene extinction of mammoths, giant sloths, dire wolves, and more in the Americas.