Availability means having timely access to information. For example, a disk crash or denial-of-service attacks both cause a breach of availability. Any delay that exceeds the expected service levels for a system can be described as a breach of availability. An information system that is not available when you need it is at least as bad as none at all. It may be much worse, depending on how reliant the organization has become on a functioning computer and communications infrastructure.
Availability, like other aspects of security, may be affected by purely technical issues (e.g., a malfunctioning part of a computer or communications device), natural phenomena (e.g., wind or water), or human causes (accidental or deliberate).
While the relative risks associated with these categories depend on the particular context, the general rule is that humans are the weakest link. (That is why each user's ability and willingness to use a data system securely are critical.)