Denial of service
Also known as DoS, DDoS
Overwhelming a system with traffic or requests so legitimate users cannot reach it.
Distributed attacks use many compromised machines at once, and rentable services have made launching one cheap. For most small and mid-sized businesses the risk is to public-facing websites and internet connectivity rather than internal systems.
Mitigation happens upstream, at the hosting provider or a scrubbing service, because once the traffic reaches your connection the bandwidth is already consumed. Your own firewall cannot help with a saturated pipe.
Where this comes up
Denial of service sits inside our cybersecurity and threats work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.