I’d start with the following, and refine if necessary:

“Gaining unauthorized access to a protected computer resource by technical means.”

• Port scanning --> Not hacking because there isn’t any access to resources gained*
• Using default passwords that weren’t changed --> Not hacking because the resource wasn’t protected*
• Sending spam --> Not hacking because there isn’t any access to resources gained
• Beating the admin with a wrench until he tells you the key --> Not hacking because it’s not by technical means.
• Accessing teacher SSN’s published on the state website in the HTML --> Not hacking because the resource wasn’t protected, and on the contrary was actively published**
• Distributed denial of service attack --> Not hacking because there isn’t any access to resources gained

* Those first two actually happened in 2001 here in Switzerland when the WEF visitors list was on a database server with default password, they had to let a guy (David S.) go free
** The governor and his idiot troupe eventually stopped their grandstanding and didn’t file charges against Josh Renaud of the St. Louis Post-Dispatch reporter, luckily

I haven’t heard of a firewall failing open when overwhelmed yet. Usually quite the opposite, a flood disables access to more than just the targeted device, when the state table overflows.

But maybe there is a different mechanism I’m not aware of. How would the DDoS change the properties of ingress?

DDoS is not hacking