I don't think there is a precise definition but it seems that when most people here refer to a transport they mean a device that picks up the digital signal from wherever it is stored and passes it along to another device, still as a digital signal, where the conversion from digital to analog takes place.
I looks as if the output of the Onkyo dock is analog, meaning the conversion is happening within the iPod or the dock but I doubt anyone would quibble much with your decision to call it a transport.