The four principal dimensions to dependability are:
1. Availability: Informally, the availability of a system is the probability that it will be up and running and able to deliver useful services to users at any time.
2. Reliability: Informally, the reliability of a system is the probability, over a given period of time, that the system will correctly deliver services as expected by the user.
3. Safety: Informally, the safety of a system is a judgment of how likely it is that the system will cause damage to people or its environment.
4. Security: Informally, the security of a system is a judgment of how likely it is that the system can resist accidental or deliberate intrusions.