An occurrence is something that happens or takes place, especially an event or incident worth noting. When a teacher asks if there were any unusual occurrences during recess, she's asking if anything noteworthy happened. Scientists studying earthquakes track each occurrence carefully to understand patterns.

The word often suggests something that happens more than once or that you're paying attention to. A single rainstorm is an event, but if you're studying weather patterns, each storm becomes an occurrence you record and analyze. You might notice the occurrence of a particular bird in your backyard every spring, or track the occurrence of spelling mistakes in your writing to see what patterns emerge.

Occurrence carries a slightly formal tone. You probably wouldn't say “That was a weird occurrence” about dropping your pencil, but you might use it to describe something more significant: “The occurrence of three fire drills in one week made everyone wonder if something was wrong with the alarm system.”