The Jargon File (version 4.4.7, 29 Dec 2003):

NSP
 /N·S·P/, n.

    Common abbreviation for ‘Network Service Provider’, one of the big national
    or regional companies that maintains a portion of the Internet backbone and
    resells connectivity to ISPs. In 1996, major NSPs include ANS, MCI,
    UUNET, and Sprint. An Internet wholesaler.