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As in my title question. Do they mean a specific region of the US, something else?
General American is a name that sociolinguists often use for the accent that Americans tend to perceive as "neutral" or non-regional.
In addition to Cronkite, a native of Missouri, a number of other early U.S. television newscasters hailed from the Midwest or the Great Plains states, such as John Cameron Swayze (Kansas), Eric Sevareid (North Dakota), and Chet Huntley (Montana). Huntley also attended college in Washington state, as did Spokane native Edward R. Murrow; the standard Pacific Northwest accent closely resembles General American.