Answer
How much recent data you can afford to lose in a disaster, measured in time, is the RPO. How long you can afford to be down is the RTO.
Also accepted: recovery point objective for RPO; recovery time objective for RTO.
Why
Nightly backups mean an RPO of up to a day; asynchronous replication, seconds; synchronous replication, zero. RTO depends on how automated the failover or restore is.