upon
prep.prep. a more formal way to say 'on'. You use it when something is on top of something else or when one thing happens right after another.
prep. on or onto something; also used to indicate that one event occurs immediately after another. More formal in register than 'on' and frequently used in literary or academic contexts.
The cat sat upon the wooden fence.
Upon arriving at the hotel, the guests were greeted with a warm drink and a map of the city.
The success of the entire operation depends upon the precise coordination of the ground teams and the local authorities during the first hour.
Etymology tree Proto-Indo-European *upó Proto-Germanic *ub Proto-Germanic *upp Proto-West Germanic *upp Old English upp Proto-Indo-European *h₂en-der. Proto-Germanic *an Proto-West Germanic *ana Old English on Old English uppan Middle English upon English upon From Middle English upon, uppon, uppen, from Old English upon, uppon, uppan (“on, upon, up to, against, after, in addition to”), equivalent to up (“adverb”) + on (“preposition”). Cognate with Old Saxon uppan (“upon”), Old High German ūfan, ūffan (“upon”), Icelandic upp á, upp á (“up on, upon”), Swedish uppå (“up on, upon”) (thence Swedish på), Danish på (“up on, upon”), Norwegian på (“up on, upon”).
Often used in fixed expressions like 'once upon a time' or after certain verbs like 'depend', 'rely', or 'insist'.
He put the book upon of the tableHe put the book upon the tableUpon is a preposition and does not require 'of' to follow it.