Redundancy
Etymology
From Latin redundare, "to overflow, be in excess" (re-, plus unda, "wave") — literally a surging over of a wave. It long meant simple surplus or needless repetition. Engineering and information theory gave it a positive technical sense: extra capacity held in reserve precisely so that failure in one place does not bring the whole down.
noun · /rih-DUN-dun-see/
The deliberate duplication of parts or capacity so that a system keeps working even when one component fails.
What it names
Why the spare, the backup, and the second route feel wasteful right up until the day the main one fails and they quietly save you.
Example
Keeping a paper copy of the boarding pass felt like pointless redundancy — until his phone died at the gate.
Related idea
Redundancy is one means to an end; robustness is the end itself — the broader ability of a system to keep functioning under stress and failure.