Other forms: occurred, occurs, occurring
Occur is the formal word for happen. Accidents occur, errors occur when a file is missing, the fault occurred twice during testing — this is the word of reports, manuals and news bulletins. At the kitchen table you say what happened.
It has a second life that has nothing to do with events: something occurs TO you, meaning it arrives in your mind. 'It never occurred to me that he was lying.' That construction is fixed — a thought occurs to a person — and it is the commonest way the word shows up in speech. Scientists use a third sense for where things are found: the plant occurs only on limestone. Occur doubles its R before endings — occurred, occurring — and the noun is occurrence, with two Cs, two Rs and an E where most people write an A. It never takes an object: things occur, but you cannot occur something.
1verboccurTo happen or take place.
“The explosion occurred shortly after midnight.”
formal in this meaning — an editor’s judgement
2verbmindTo come into someone's mind, as a thought or idea.
“It occurred to her, halfway home, that she had left the oven on.”
3verbfoundTo be found or exist in a particular place.
“The mineral occurs naturally in only a handful of valleys.”
formal in this meaning — an editor’s judgement
Also accepted in games: happen
Occur comes from Latin occurro, to run to meet, from ob-, against, and curro, to run — the same curro behind current and courier. Its first English meaning was to meet somebody in argument. Events that occur are still running towards you, and a thought that occurs to you has run up and tapped you on the shoulder.
Proto-Indo-European
*ḱers-
Latin
occurrō — run to meet, run against, befall, present itself
Middle French
occurrer
English, today
occur
Built from
Originally "meet (in argument)", borrowed from Middle French occurrer, from Latin occurrō (“run to meet, run against, befall, present itself”) from prefix ob- (“against”) + verb currō (“run, hurry, move”).
Etymology from Wiktionary, CC BY-SA