Something added.
Supplemental or added to something.
An act of falling back.
Pulverised material that falls back to earth after a nuclear explosion; fallout.
A backup plan or contingency strategy; an alternative which can be used if something goes wrong with the main plan; a recourse.
A reduction in bitumen softening point, sometimes called refluxing or overheating, in a relatively closed container.
That can be resorted to as a fallback.