We’ve heard of an ISP, but what is an NSP?
NSP is the abbreviation for Network Service Provider, which is a large company or organization that maintains a portion of the Internet backbone. That service is then resold to ISPs (Internet Service Providers) who then resell Internet access to end users.
End users normally do not have direct connections to the Internet backbone, so they must go through an ISP, which obtains its Internet access through an NSP.
When the Internet was beginning, there used to be a distinction between NSPs and ISPs, but today, the line between the two is blurring as many NSPs that manage the Internet backbone, such as AT&T, also offer ISP services directly to its customers.
Sources: Networking dictionaries and textbooks.