Apple does have a monopoly on devices that use the iPod connector. This monopoly is enforced by patent 7632114 B2.
That being said, the term monopoly doesn't mean much on its own. There is an alternative to everything.
That being said, the term monopoly doesn't mean much on its own. There is an alternative to everything.
That substitutability is defined in a distributed way isn't a well-formed argument against tying it down in any particular case. Legal courts make these determinations every day concerning the meanings of texts, whose intent can be similarly hard to divine.
"Monopolies are thus characterised by a lack of economic competition for the good or service that they provide and a lack of viable substitute goods." - via wikipedia, with book citation: http://en.wikipedia.org/wiki/Monopoly
Please don't forget about the last bit. There are a million (bad) substitutes for an iPod, regardless of how they end up connected to the computer. The word monopoly should be used with economic rigor or not at all - even economists tread lightly and discuss heavily before labeling any entity a monopoly.
Setting aside the above, the definition suffers for ignoring the question of why there are no viable substitutes. It's a dangerous omission since the implication, at least with respect to legal policy, is that it is the "fault" of the firm.
The closest one could get to a useful definition of monopoly would be: a firm whose would-be competitors are coercively excluded from competing in the market. Of course even that still suffers from the subjectivity of defining the scope of "the market".