In the context of NIST SP800-61, a precursor is an event that indicates the potential occurrence of an incident. When an organization adjusts its security stance in response to online threats made by a known hacktivist group, the initial event-the threats-would be considered a precursor. It is an indication of a potential future attack or security incident34.
Reference:
NIST SP 800-61 Rev. 2, Computer Security Incident Handling Guide3.
Computer Security Incident Handling Guide - NIST