It is an entity or event with the potential to adversely impact a system through...

A threat is at any circumstance or event with the potential to adversely impact organizational operations (including mission, functions, image, or reputation), organizational assets, or individuals through an information system via unauthorized access, destruction, disclosure, modification of information, and/or denial of service. Also, the potential for a threat-source to successfully exploit a particular information system vulnerability.

References: https://en.wikipedia.org/wiki/Threat_(computer)