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