Is the National Aviation Authority the same as the Civil Aviation Authority?
The terms National Aviation Authority and Civil Aviation Authority are indeed two generic terms to describe the same thing - the government organisation responsible for managing non-military aviation within a country.

It's worth noting that the specifics of how the organisation works and what it regulates is down to each country, so the terms are broad and generic. For example, Iceland doesn't have any kind of specific CAA - rather, those roles are performed by the Icelandic Transport Authority.

In addition, it wouldn't really be right to refer to a countries authority as the National Aviation Authority or the Civil Aviation Authority (Unless that's what they've called themselves). 

For example, the FAA is an example of a National Aviation Authority, it isn't known as the National Aviation Authority. The UK's Civil Aviation Authority is the Civil Aviation Authority in the UK and an example of a National Aviation Authority