An accident is something going wrong unexpectedly. Physical examples include an unintended collision (including a person or object unintendedly falling), getting injured by touching something sharp, hot, electrically live, ingesting poisons, or getting injured by not properly landing when jumping. Non-physical examples include babies being born.
Technically, "accidents" do not include incidents where someone is at fault, i.e., negligent: where someone fails to take reasonable precautions in the circumstances. If the results of such negligence were foreseeable, they were certainly not "accidental" at that level, and the negligent person can be held liable for damages and personal injuries. In an "accident", there is simply nobody to blame, because the event was unforeseeable or very unlikely. For example, a pharmacist negligently mixes the wrong chemicals and mislabels them for sale; a person ingesting the chemicals according to the label instructions has been "accidentally" poisoned, but the pharmacist's mistake was not so accidental as much as it was negligent.
A common misconception is that a gun can "go off" accidentally, where in truth, such gun accidents are extremely rare, and most gun injuries are caused by intentional acts that create the hazard of injury (i.e., pulling the trigger of a loaded gun). A defective gun that fires when dropped could qualify as being "accidental", however, one would still have to examine the cause for the gun being intentionally loaded and being handled carelessly.
Often accidents are investigated so that we can learn how to avoid them in the future. This is sometimes called root cause analysis, but does not generally apply to accidents that cannot be predicted with any certainty. For example, a root cause of a purely random incident may never be identified, and thus future similar accidents remain "accidental." Modified from Wikipedia's article licensed under GNU FDL Template:Safety