Imagic was a company formed from ex Atari employees back in the early 1980s. They made some of the best third party games available for the Atari 2600. Almost all their games were of very high quality, and games like Atlantis and Quick Step! went on to be classics in their own right.
They were very dedicated to their fanbase and often held gaming contests, like the Atlantis/Atlantis II World Championship. They even had a high score club called the Numb Thumb Club that would send out prizes to high scoring players. This was very similar to the well known Activision Club from Activision. They reportedly offered a $1000 USD cash prize to the first player who could complete their "Riddle of the Sphynx" title.
Imagic eventually went out of business because they made no attempt to go with the changing market. They stayed with the Atari 2600 instead of moving up and producing titles for the Sega Master System and Nintendo Entertainment System.
Imagic titles usually came in a distinctive shiny silver box. These boxes were usually featured artwork that was far beyond the quality of the graphics on Atari's own boxes. Their cartridges were shiny as well. A few years later Nintendo took that shiny cartridge idea from Imagic when they released The Legend of Zelda.
Imagic also produced a few Atari 2600 related products, such as the popular Video Storage Center, which was a wood grained case that held your Atari console, controllers and games (I used to have one of those).
Gamers who live in South America may occaisionally encounter multi-game cartridges labeled with the Imagic name. Those games were not made by Imagic, they are bootlegs, and the bootleggers just used the popular Imagic name to appear more legitimate.