A change that must be implemented as soon as possible without strictly following the standard process e.g. to resolve an incident or implement a security patch.
The process for assessment and authorization is expedited to ensure quick implementation, so scheduling and documentation is not a priority.
The change authority may be separate from what is standard or normal practice, typically smaller in number but with greater capacity to expedite approval.
https://www.bmc.com/blogs/itil-change-enablement/