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A sudden, unwanted, and unplanned event or event sequence that has led to harm to people, the environment, or other tangible assets.

By this definition, we have moved from talking about the future to considering the past. An accident is an event that actually has caused harm to one or more assets. The definition further implies that an accident is not predictable with respect to whether and when it occurs. The definition emphasizes that an accident is a distinct event or event sequence and not a long‐term exposure to some hazardous material or energy. Suchman (1961) argues that an event can be classified as an accident only if it is unexpected, unavoidable, and unintended.

Accidents can be classified in many different ways, such as according to types of accidents, causes of accidents, and severity of accidents. Some terms that are used to describe accidents are, for example, major accident, process safety accident, personal accident, occupational accident, and disaster. In many cases, the accident types are not clearly defined or the definitions may vary from case to case.

In the process industry, it is common to distinguish between process safety accidents and personal accidents. Process safety accidents are related to the process plant as such, the processes going on and the materials being used in the plant. Common causes of these accidents are that the process comes out of control or that hazardous substances are released. The potential consequences can be very large, both for people, the environment, and other assets. Personal accidents or occupational accidents, usually involve one or few people. Typical examples are falls, cuts, crushing, and contact with electricity. In this case, the categorization is done mainly with respect to the types of accidents (and thereby also causes). In practice, the categorization is also according to the degree of possible consequences that may occur.

Of particular concern are accidents with very large consequences. These accidents – called major accidents – receive a lot of attention, with thorough investigation of causes and sometimes with wide impact on regulations, technology, operations, and public perception of risk. As mentioned previously, process safety accidents is the equivalent term being used in the process industry. There is no generally accepted definition of what this is, but sometimes the term high‐impact, low‐probability event neatly summarizes the main features: they are events of low probability that have a high impact both directly (in terms of direct consequence) and indirectly (e.g. in terms of regulatory and political implications).

Accidents and accident models are discussed in more detail in Chapter 8.