Pronunciation: (u-pon', u-pôn'), [key]
1. up and on; upward so as to get or be on: He climbed upon his horse and rode off.
2. in an elevated position on: There is a television antenna upon every house in the neighborhood.
3. in or into complete or approximate contact with, as an attacker or an important or pressing occasion: The enemy was upon us and our soldiers had little time to escape. The Christmas holiday will soon be upon us and we have hardly begun to buy gifts. The time to take action is upon us.
4. immediately or very soon after: She went into mourning upon her husband's death.
5. on the occasion of: She was joyful upon seeing her child take his first steps.
6. on (in any of various senses, used as an equivalent of on with no added idea of ascent or elevation, and preferred in certain cases only for euphonic or metrical reasons): He swore upon his honor as a gentleman.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.