Disaster recovery is the process of rebuilding an application, from scratch or from backups, to restore full functionality after some adverse event.
The word resilience is applicable. A web application should be resilient - which is different from disaster recovery. Resilience implies that the application continues functioning in the face of adversity, in the event of (possibly multiple) internal failures. Being able to defend against a DDOS attack is a form of resilience. With automatic scaling, an application that can change its resource allocation would be able to withstand higher or significantly higher traffic, as is the case with an attack. Having fail-over load balancers is another example. A third example is maintaining several database replicas, such that failure of one does not affect operation.