Well in theory it was an oscilliscope. The first game (Pong) was designed on an Oscilliscope. (Pong reached the arcades in 1972) - note the original game was created in 1958 and was called Tennis for Two - but this was never commercially sold.
The first commercial home based video games system was designed by Magnavox and called the Odyssey - it came out in 1972.
The first arcade game was Computer Space by Nutting Associates (1971)