According to Wikipedia, an impact event is the collision of an asteroid, comet, meteoroid, or other celestial object with another celestial object such as Earth.
This issue has been in the news a lot this week. First, the BBC's article "Meteorites injure hundreds in central Russia:"
A meteor crashing in Russia's Ural mountains has injured at least 950 people, as the shockwave blew out windows and rocked buildings. Most of those hurt, in the Chelyabinsk region where meteorites fell, suffered cuts and bruises but at least 46 remain in hospital. A fireball streaked through the clear morning sky, followed by loud bangs.And yesterday we had a near miss with an asteroid:
An asteroid as large as an Olympic swimming pool has raced past the Earth at a distance of just 27,700km (17,200mi) - the closest ever predicted for an object of that size. It passed far closer even than the geosynchronous satellites that orbit the Earth, but there was no risk of impacts or collisions.
(From the BBC article "Asteroid 2012 DA14 in record-breaking Earth pass.")
What would happen if a large celestial object hit the earth? That's the subject of Larry Niven and Jerry Pournelle's novel Lucifer's Hammer:
The gigantic comet had slammed into Earth, forging earthquakes a thousand times too powerful to measure on the Richter scale, tidal waves thousands of feet high. Cities were turned into oceans; oceans turned into steam. It was the beginning of a new Ice Age and the end of civilization. But for the terrified men and women chance had saved, it was also the dawn of a new struggle for survival--a struggle more dangerous and challenging than any they had ever known....
NB: The novel was written back in the 1970s and is, at times, racist. Aside from that major flaw, it's the best Earth-after-comet-cataclysm novel I've ever read.