According to Wikipedia: "The Atari 2600, released in 1977, is the first successful video game console to use plug-in cartridges instead of having one or more games built in. Originally known as the Atari VCS—for Video Computer System—the machine's name was changed to "Atari 2600" (from the unit's Atari part number, CX2600) in 1982, after the release of the more advanced Atari 5200. It was wildly successful, and during the 1980s, "Atari" was a synonym for this model in mainstream media."
Here's a cool commercial from the golden age of video games: