Learner's definition of UPON
1 : on
He carefully placed the vase upon the table.
They built their city upon a cliff overlooking the sea.
She was seated upon a throne.
an assault upon traditional values
She was admitted to his office immediately upon her arrival.
That kind of behavior is frowned upon.
2 — used to say that someone or something is very close or has arrived
The enemy was suddenly upon us.
The holidays are nearly upon us. [=the holidays are nearly here; the holidays have nearly begun]
Before we knew it, the date of the wedding was upon us.
3 — used to emphasize something that is repeated many times
We removed layer upon layer [=many layers] of old paint from the paneling.
She studied for hour upon hour. [=for many hours]
thousands upon thousands of people