Content decay is what happens to a page that ranked well two years ago and now brings in a third of the traffic it used to. Nothing broke. The page simply aged while the query around it moved.
The causes separate cleanly. Facts go stale, so a page citing 2023 pricing loses to one citing this year's. Intent shifts, and a query that wanted a definition now wants a comparison. Competitors publish more complete coverage. Or the SERP itself changed shape, and the answer moved into a snippet above the results.
Spotting it needs a comparison over time rather than a snapshot. Pull Search Console data for two matching periods, six months apart, and sort pages by absolute click loss. The pages at the top of that list are where a refresh pays best, because they already have links and history working for them.
The fix is usually an update rather than a new page. Refresh the data, cover what the current SERP covers, keep the URL, and note the change date. Publishing a second page on the same topic tends to create cannibalisation instead of recovery.