Threat Agent (TA) – An entity or experience that exposes a system to a loss.
The TA needn’t be cognizant as an earthquake can topple a data center without forethought or fire consumes building and paperwork without prejudice. A TA may also be a hacker or hackers phishing for information.
Vulnerability (V) – A characteristic that exposes something to a weakness without a countermeasure to mitigate potential losses.
A warehouse may have a vulnerability to fire, a data center may have a vulnerability to a physical threat like an earthquake or network may have a vulnerability around access because it lacks a firewall (or perhaps a well-managed and patched firewall).