The Free On-line Dictionary of Computing (19 January 2023):

ISP

   1. Internet Service Provider.

   2. Instruction Set Processor.


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

ISP
 /I·S·P/

    Common abbreviation for Internet Service Provider, a kind of company that
    barely existed before 1993. ISPs sell Internet access to the mass market.
    While the big nationwide commercial BBSs with Internet access (like America
    Online, CompuServe, GEnie, Netcom, etc.) are technically ISPs, the term is
    usually reserved for local or regional small providers (often run by
    hackers turned entrepreneurs) who resell Internet access cheaply without
    themselves being information providers or selling advertising. Compare NSP
    .