Process safety is an interdisciplinary engineering domain focusing on the study, prevention, and management of large-scale fires, explosions and chemical accidents (such as toxic gas clouds) in process plants or other facilities dealing with hazardous materials, such as refineries and oil and gas (onshore and offshore) production installations. Thus, process safety is generally concerned with the prevention of, control of, mitigation of and recovery from unintentional hazardous materials releases that can have a serious effect to people (onsite and offsite), plant and/or the environment.

The American Petroleum Institute defines process safety as follows:

A disciplined framework for managing the integrity of hazardous operating systems and processes by applying good design principles, engineering, and operating and maintenance practices. It deals with the prevention and control of events that have the potential to release hazardous materials or energy. Such events can cause toxic effects, fire or explosion and could ultimately result in serious injuries, property damage, lost production, and environmental impact.

The same definition is given by the International Association of Oil & Gas Producers (IOGP). The Center for Chemical Process Safety (CCPS) of the American Institute of Chemical Engineers (AIChE) gives the following:

A discipline that focuses on the prevention of fires, explosions, and accidental chemical releases at chemical process facilities.

Process safety scope is usually contrasted with occupational safety and health (OSH). While both domains deal with dangerous conditions and hazardous events occurring at work sites and/or while carrying out one's job duties, they differ at several levels. Process safety is primarily concerned with events which involve hazardous materials and are or have the potential to escalate to major accidents. A major accident is usually defined as an event causing multiple fatalities, extensive environmental impact, and/or significant financial consequences. The consequences of major accidents, while typically limited to the work site, can overcome the plant or installation boundaries, thus causing significant offsite impact. In contrast to this, occupational safety and health focuses on events that cause harm to a limited number of workers (usually one or two per event), have consequences limited to well within the work site boundaries, and do not necessarily involve unintended contact with a hazardous material. Thus, for example, a gasoline storage tank loss of containment resulting in a fire is a process safety event, while a fall from height occurring while inspecting the tank is an OSH event. Although they may result in far higher impact to people, assets and the environment, process safety accidents are significantly less frequent than OSH events, with the latter account for the majority of workplace fatalities. However, the impact of a single major process safety event on such aspects as regional environmental resources, company reputation, or the societal perception of the chemical and process industries, can be very considerable and is usually given prominent visibility in the media.

The pivotal step in a process safety accident, around which a chain of accident causation and escalation can be built (including preventative and control/mitigative safety barriers), is generally the loss of containment of a hazardous material. It is this occurrence that frees the chemical energy available for the harmful consequences to materialize. Inadequate isolation, overflow, runaway or unplanned chemical reaction, defective equipment, human error, procedural violation, inadequate procedures, blockage, corrosion, degradation of material properties, excessive mechanical stress, fatigue, vibration, overpressure, and incorrect installation are the usual proximate causes for such loss of containment. If the material is flammable and encounters a source of ignition, a fire will take place. Under particular conditions, such as local congestion (e.g., arising from structures and piping in the area where the release occurred or the flammable gas cloud migrated), the flame front of a flammable gas cloud can accelerate and transition to an explosion, which can cause overpressure damage to nearby equipment and structures and harm to people. If the released chemical is a toxic gas or a liquid whose vapors are toxic, then a toxic gas cloud occurs, which may harm or kill people locally at the release source or remotely, if its size and the atmospheric conditions do not immediately result in its dilution to below hazardous concentration thresholds. Fires, explosions, and toxic clouds are the main types of accidents with which process safety is concerned.

In the domain of offshore oil and gas extraction, production, and subsea pipelines, the discipline of process safety is sometimes understood to extend to major accidents not directly associated with hazardous materials processing, storage, or transport. In this context, the potential for accidents such as ship collisions against oil platforms, loss of FPSO hull stability, or crew transportation accidents (such as from helicopter or boating events), is analyzed and managed with tools typical of process safety.