1. 交通事故；车祸
    An accident happens when a vehicle hits a person, an object, or another vehicle, causing injury or damage.

accident

    e.g. She was involved in a serious car accident last week...
           上星期她卷入了一场严重的车祸。
    e.g. Six passengers were killed in the accident.
           六名乘客在车祸中丧生。

2. （不幸的）意外遭遇，不测事件;事故
    If someone has an accident, something unpleasant happens to them that was not intended, sometimes causing injury or death.

    e.g. 5,000 people die every year because of accidents in the home...
           每年有5,000人死于家中的意外事故。
    e.g. The police say the killing of the young man was an accident.
           警方说这个年轻人被杀是一个意外。

3. 意外；偶然
    If something happens by accident, it happens completely by chance.

    e.g. She discovered the problem by accident...
           她碰巧发现了这个问题。
    e.g. Almost like an accident of nature, this family has produced more talent than seems possible.
           就像是天缘巧合，这个家族不可思议地人才辈出。

4. 可能发生意外的事物（或人）
    If you describe something or someone as an accident waiting to happen, you mean that they are likely to be a cause of danger in the future, for example because they are in poor condition or behave in an unpredictable way.

    e.g. A lot of the city's buildings are accidents waiting to happen.
           该城市中许多楼房都存在事故隐患。

5. 并非偶然;事出有因
    You begin a sentence with 'it's no accident' if you want to suggest that something was done deliberately or has a logical explanation, although it might give the impression of having happened by chance.

    e.g. It's no accident that the boom in police series on TV coincided with the decline of the Western.
           警匪剧在电视上盛行之时恰好是西部片衰退之际，这并非偶然。