Short version: Ad fatigue is a decline in ad performance that can happen when the same audience sees the same or very similar creative too often.
People may stop noticing the ad, scroll past it faster, or become less responsive to the message. In performance data, ad fatigue can appear as falling click-through rate, rising cost per click or cost per acquisition, weaker conversion volume, or lower engagement. Those signals are clues, not proof by themselves, because seasonality, targeting, budget changes, landing page issues, and offer strength can also affect results.
The practical fix is not always to increase spend or replace everything at once. Teams usually refresh hooks, rotate formats, test new visuals, adjust frequency, segment audiences, or change the offer angle while keeping the campaign objective clear. Creative variety matters because repeated exposure can help memory up to a point, then become wasteful or annoying.
Related terms: ad creative, attribution, and campaign objective.