Correct spelling, explanation: incident, like many English words, has its roots in Latin, specifically in the word incidere, therefore the spelling remained relatively unchanged. When contemplating the spelling of similar words of Latin origin, it is worth remembering that the original consonants rarely change, which is also the case in incident.

Definition of incident:
Noun
1. An occurrence; a happening of something, usually negative
Since the incident at the bus station, I tend to avoid that place.
Police look for information after a person was shot in a possible road rage incident.

Incorrect spelling, explanation: incident comes from Latin incidere, which explains its spelling. In the case of insident, the spelling may look convincing as it fits the pronunciation quite well, but in this case c is also read as s. So it may be tricky, but insident is definitely not a correct word.