While I've seen varying descriptions, in general, any resource can fall under such an Umbrella, including network bandwidth. If someone's saturating your uplink with garbage downlink traffic, there's very little mitigating it on your network perimeter would really accomplish anyway.
In a more traditional sense, we can't mitigate against a DDoS consuming CPU or memory either; processing that many inbound packets through our IPS and inbound firewall is always going to consume both.
Proper DDOS protection requires multiple routing paths and large amounts of WAN bandwidth, far to expensive to do on your own.
I had a gaming server years ago that got DDOS because of an unhappy user ( someone killed him in game and took all of his stuff) and even having it hosted by a large gaming website their DDOS protection was useless because it turns out they were doing it themselves.