In computer security a threat is a possible danger that might exploit a vulnerability to breach security and thus cause possible harm.
A threat can be either "intentional" (i.e., intelligent; e.g., an individual cracker or a criminal organization) or "accidental" (e.g., the possibility of a computer malfunctioning, or the possibility of an "act of God" such as an earthquake, a fire, or a tornado) or otherwise a circumstance, capability, action, or event
National Information Assurance Glossary defines threat as:
Any circumstance or event with the potential to adversely impact an IS through unauthorized access, destruction, disclosure, modification of data, and/or denial of service. (Source: wikipedia.org)