upon

adverb

1. Being the target of an action.

- He was set upon by the agitated dogs

2. Incidental to a specified point in time or order of action; usually combined with here-, there- or where-.

- The clock struck noon, whereupon the students proceeded to lunch.

preposition

1. Being above and in contact with another.

- Place the book upon the table.

2. Being directly supported by another.

- The crew set sail upon the sea.

- She balanced upon one foot.

3. Being followed by another so as to form a series.

- hours upon hours, years upon years

4. At a prescribed point in time.

- The contract was rendered void upon his death.

5. On.