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.

(construction) 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.

Misspelling of fall back.

That can be resorted to as a fallback.