United States radio and TV network, founded as such in 1943.
ABC traces its history to the early days of radio, when it was actually the Blue Network of NBC. In 1942, RCA (under pressure from the FCC) spun off the Blue assets as the Blue Network, Inc. In 1943, Edward Noble, the Lifesavers tycoon, purchased Blue Network, and in 1945 changed its name (after acquiring the name from George Storer).
ABC's radio properties were competent, but in the world of TV, it was something of a latecomer. With limited capital (and the likes of its former parent NBC, CBS and DuMont to deal with), its entry into the TV arena was slow. In 1954, two much-needed shots in the arm came, the first being from Noble's sale of the network to United Paramount Theaters (which, like most of the theater chains in the 1950s, had been divested from its parent due to antitrust concerns), and the second from striking a deal with Walt Disney to invest in his coming theme park, and to produce a TV show around it. The park and the show were called Disneyland.
Even with this help, ABC's TV ventures were still slow to come to the top, despite rosters of quality shows (and quite a few memorable ones, like Ozzie and Harriet). Through the 1950s and 1960s, ABC TV usually came in third place (behind CBS and NBC; by this time, DuMont had disappeared).
In 1968, there was considerable shakeup at ABC. Their radio networks were split up into four separate operations: ABC Information (news and talk), ABC Entertainment (old-style radio shows etc.), ABC Contemporary (Top 40), and ABC FM. Also, on the TV side, programming executives were pioneering the use of demographics to count ratings points, a move that would change the TV industry forever.
Another ABC TV first was its use of innovative advertising techniques. Starting in 1970 and rolling right on into the early 1990s, their promotions department green-lit huge, big-budget music videos, working on a scale that predated MTV by over a decade. Though things were off to a slow start, clever programming by Michael Eisner and others bridged the gap. Finally, in 1975-1976, the strategy paid off: ABC was #1 in the ratings for the first time ever.
ABC's popularity remained solid (though not always tops) through the late 1970s and 1980s, even as their shows aged. They were able to avoid the ratings debacle that befell NBC (and almost destroyed it) in 1979-1980, and in 1986 they merged with station owner Capital Cities. By this time, their popularity had cooled, only to rise again with shows like Full House and Roseanne two years later
In 1995, Disney acquired ABC, and Capital Cities with it. As of now, ABC Radio is one of only three major radio networks still in existence (the others being Clear Channel and Westwood One), and their TV division is still doing reasonably well; in 1999-2000, Who Wants to be a Millionaire? took them from 3rd place to 1st -- another first for them.
Sources: Some of this info came from http://www.oldradio.com, and some was verified using TVparty (http://www.tvparty.com).