The "i" is coming from the iMac and the iBook, after the return of Steve Jobs in 1998 the iMac and the iBook were created in order to reduce the line of production to only a few machines following the simple grid:
CONSUMER | PRO
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DESKTOP | iMac | PowerMac
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PORTABLE | iBook | PowerBook
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as a consequence the iPod and the iPhone were called like that, iPod because had a chassis like a pod behind, the iPhone cause it was reinventing the phone. iPad too, closing the trinity of the 00's first decade of Apple products. 2001-2007-2010.