Apple used i, for Internet. To say: Internet Macintosh, which the marketing team said it was a stupid name, but Steve Jobs insisted in having that name. So, we have iMac. And this is how it all started. I is also used for other names like individual.
The reason why it was called Internet Macintosh, is because it was the first Apple home computer that you could go on the internet with, without any extra addon, simply, 'out of the box' (you need to get ISP service, of course) and all being on the low cost, but sufficient power to go online, watch videos, and such