An extranet is essentially the legacy term for what we now call a customer portal or partner portal. The word was widely used in the late 1990s and early 2000s to describe private networks that extended internal corporate systems to external parties (customers, suppliers, distributors).
In modern usage, “extranet” still appears in enterprise contexts, government, and education. Day-to-day, “customer portal,” “client portal,” “partner portal,” or “supplier portal” have largely replaced it. The technology has evolved from VPN-based access to modern cloud-based, web-accessible portals.
See also: intranet, customer portal.