The vast network of suburbs, small towns, strip malls, and highways surrounding Chicago, Illinois. Chicagoland could be said to be a small state in its own right, much like the region of southern Oregon and northern California wishfully known by residents as Jefferson State. The area itself is made up of the eastern 3/4 of the north half of Illinois, stretching west to De Kalb and as far south as Bourbonnais, plus Gary, Indiana and a few towns in southeastern Wisconsin.
The region contains a mixture of urban and suburban sprawl, plus commuters. The few surviving acres of farmland around its edges are rapidly being eaten by housing developments. The Chicago Tribune is the paper and deep dish pizza the cuisine of choice. Many residents look on Chicago proper as the center of the earth. People do not say "I live in Naperville" or "I'm from Schaumburg", but "I'm from the south side." Everything wants to be identified with Chicago. We aren't some suburb; we are Chicago.
Television is probably the one thing that has defined "Chicagoland" most for me, strangely enough. WGN broadcasts to the whole Chicagoland area. O'Connor Ford and Empire Carpet commercials hawk wares not to their own city, but to Chicagoland, land of Chicago. This is the land that grew out of a swamp to produce the third largest city in the nation, the most important inland port. This is the land of business and money, which grew out of its dying farms. This is the land that supports Chicago, that buys its products and watches its programs and transports its foodstuffs.
There is an interesting dualism of identity that most people don't quite want to admit. We are a city; we are a land. A citified country; a country of cities. In name we are both.