A change that must be implemented as soon as possible without strictly following the standard process e.g. to resolve an incident or implement asecuritypatch.

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/