[w]hen action(s) result in the inability to communicate and/or the inability of an AIS or any essential part to perform its designated mission, either by loss or degradation of a signal or operational capability.
Denial of service (DoS) occurs
when an entity fails to perform its function or prevents other entities from performing their functions, which may be a denial of access, a denial of communications, a deliberate suppression of messages to a particular recipient, a fabrication of extra traffic, an MTA was caused to fail or operate incorrectly, an MTS was caused to deny a service to other users. Denial of service threats include the following: denial of communications, MTA failure, MTSflooding.
↑NICCS, Explore Terms: A Glossary of Common Cybersecurity Terminology (full-text).