Banner blindness is a phenomenon of habituation by which web users unconsciously ignore usual ad placements on a web page.
Banner blindness is one of the main reasons for downward trend of CTRs.
To prevent banner blindness new ad units and ad placements are created and eye catching creatives (sometimes intrusive) are used.
Banner blindness is diffrent from banner fatigue.
Banner blindness is exposed by eye tracking studies:
Image credit Altics
How a publisher tries to prevent banner blindness: