Content Decay
Also known as: Content depreciation
The gradual loss of traffic and usefulness in published content as the subject moves, competitors publish, and facts age.
Content decay is the default state of published content. Traffic to an untouched page declines over time as the subject shifts, better answers appear, and the specifics age out. A site left alone does not hold steady. It declines slowly enough to look stable for about a year.
The practical consequence is the maintenance floor: the work required to not lose ground, before any growth work counts. Most properties never clear it.
Decay is not uniform. Event-driven content has a half-life in weeks; reference content that is kept current can improve with age. Measuring decay on the whole site averages the two together and hides both.