1. Historically, the union of all the major noncommercial, academic, and hacker-oriented networks, such as Internet, the pre-1990 ARPANET, NSFnet, BITNET, and the virtual UUCP and Usenet ‘networks’, plus the corporate in-house networks and commercial timesharing services (such as CompuServe, GEnie and AOL) that gateway to them. A site is generally considered on the network if it can be reached through some combination of Internet-style (@-sign) and UUCP (bang-path) addresses. See Internet, bang path, network address.
2. Following the mass-culture discovery of the Internet in 1994 and subsequent proliferation of cheap TCP/IP connections, “the network” is increasingly synonymous with the Internet itself (as it was before the second wave of wide-area computer networking began around 1980).
3. A fictional conspiracy of libertarian hacker-subversives and anti-authoritarian monkeywrenchers described in Robert Anton Wilson's novel Schrödinger's Cat, to which many hackers have subsequently decided they belong (this is an example of ha ha only serious).
In sense 1, the network is often abbreviated to the net. “Are you on the net?” is a frequent question when hackers first meet face to face, and “See you on the net!” is a frequent goodbye.