years later; after (some) years
Used to indicate a point in time a certain number of years after a reference event. Often follows a number of years (e.g., 数年後, 10年後).
彼は10年後に帰国した。
He returned to his country ten years later.
数年後、その町は大きく変わっていた。
Several years later, the town had changed greatly.
Let's meet at this place again in three years.
We held a reunion ten years after graduation.
年後 marks a point a certain number of years after a reference event, while 以後 more broadly means 'from that point on' without specifying years.
年後 (as in 5年後) means 'years later', while 前 (as in 5年前) means 'years ago', giving the opposite time direction.
年後 specifies a number of years into the future, whereas 後日 vaguely means 'at a later date' without indicating how much time will pass.
Compound of 年 (year) and 後 (after, later).