The following are my understanding of the terms used.
Incident: Any happening/situation which is a deviation from the normal, which has a potential to develop into an accident. It is generally understood that an incident is an indication of things to come (accidents, emergency) if not addressed properly. Using Bhopal Gas Tragedy as an example - e.g. increase in pressure of the methylisocyanate storage system, pressure higher than specified safe limit
Accident: An event that has the potential to harm people (both individuals and community), property, flora and fauna etc. Normally accidents are manageable with the resources available in the organization, if acted in time. e.g. Pressure exceeded the threshold value and poisonous gas was released into the atmosphere
Emergency: An emergency is a situation (arising because the organization could not take immediate appropriate corrective action on the accident) where the organization alone cannot handle the accident situation; it needs help from other agencies outside the organization to control the situation and reduce the damage. There is a possibility of irreparable damage to human beings, property, flora, fauna etc. Under the India Law (Manufacture, Storage and Import of Hazardous Chemicals Rules), certain organizations are expected to prepare (plan) for "on-site" emergency and "off-site" emergency. e.g. Methylisocyanate leaked and spread to Bhopal City, killing thousands of people
Mock-drill: An organization which is required to have an on-site emergency plan has to conduct "mock-drills" to (a) to check if the emergency plan is working, (b) to assess and improve responses from employees, (c) to identify and plug gaps in the plan (d) to ensure that equipment required during emergencies are in good working condition and (e) the coordination with agencies outside the organization and other stakeholders is up to the mark. In India, Factories Act requires that "mock drills" are conducted wherever there is a possibility of fire in a factory.
Awareness Training: Training in EHS issues; training each employee on various signals of EHS problems and make them aware of the immediate actions to be taken by them; this includes awareness on chemicals and materials that they use, escape routes and emergency exits, emergency sirens, contacts in case of accidents, incident reporting etc. Employee awareness on these issues can reduce EHS accidents tremendously.
(Imagine, if only people in Bhopal were made aware that they should close their nose with a wet handkerchief to avoid inhaling methylisocyanate, thousands of lives could have been saved !)