Delos is the name of the theme park in 1973's Westworld and 1976's Futureworld. Similar to Disneyland or smaller theme parks, it initially contains three "worlds" or "resorts" (or smaller parks): Medieval World, Roman World and Westworld.
Delos shuts down after the "turmoil" in Westworld. Chuck Browning spins the malfunction and disaster as being contained only with in Westworld. After two years and $1.5B Delos reopens, minus Westworld, but with the addition of two new worlds: Spa World and Futureworld, having replaced "every circuit, every program, every robot."
Delos may also be the name of the company that owns/controls the theme park, but this is not made clear in the films.