New viewers

Found in the Audience tab, next to returning viewers and unique viewers. The definition is tied to the window you chose: someone who last watched two years ago counts as new in a 28-day report, then vanishes from the count once you widen the range to cover that earlier visit.

Which means comparing new viewers over 7 days against new viewers over 90 days compares nothing at all. Read the figure next to returning viewers instead. Heavy on new with almost no return usually means the video reached people who had no reason to stay.