The Sega Mega Drive was released in Japan in 1988 and It was then released in America in 1989. It was called the Mega Drive in Japan and in Europe, while in America it was called the Sega Genesis. It was called this because Sega couldn't get the rights for the Mega Drive name in the United States. The Mega Drive was the first of the 16 bit machines to achieve notable success  in Europe and America. With over 29 million sold, it became Sega's most successful console, due in part to the company's realizing that the machine needed a mascot to match the success of the Mario series. That's where Sonic the Hedgehog came in. 

Sonic the Hedgehog was a massive success in the US and Europe, and with Nintendo in no hurry to follow up it’s huge selling console, the Megadrive began to rapidly catch up to Nintendo’s lead in the US, and increase its already huge lead in the European success.