A big question, admittedly, but one which 10 or 15 years ago wouldn't have been half as difficult to answer. First and foremost, radio was a communication technology, or, in dictionary speak: "the transmission and reception of electromagnetic waves having a frequency in the range 104 to 1011 or 1012 hertz, especially those carrying sound messages" (AskOxford.com). However it also came to refer to both a physical device ("an apparatus for receiving radio programmes") and, perhaps most interestingly of all, the content broadcast to it (the OED lets me down here). Whilst the first two definitions may be more dictionary-friendly than the third, my strongest and most immediate associations with the word radio are almost all related to content.
The advent of digital distribution technologies clearly challenges the first of these three definitions. Radio is no longer an exclusively analogue technology, reliant on the transmission and reception of electromagnetic waves. It can now be broadcast in binary via a plethora of broadcast/communication technologies (e.g. DAB, IP, satellite, cable). The radio programme I am listening to as I write this (Radio 4's excellent Reith Lectures) is being delivered to me not 'over the airwaves', but via the internet, as a string of 1s and 0s.
This expansion in delivery methods is mirrored in the range of ways in which it is now possible to receive radio, which brings into question our second definition. Is my computer/television/mobile phone/MP3 player a radio because it is capable of receiving radio programmes? As multi-function devices become more and more commonplace, the notion of radio as physical apparatus becomes increasingly problematic.
So, if radio can no longer be comfortably defined as a delivery mechanism or as a physical device, that leaves us with content. Is there something intrinsic about radio content that marks it as 'radio'? The diversity of output disseminated under the banner of radio suggests not. In which case, does the producer/broadcaster get to decide what is and isn't radio? Inevitably, the digital revolution is blurring the lines here too. The nascent podcasting industry is promising a democratisation of the radio production process, wresting control of what is and isn't radio from the hands of established broadcasters.
Maybe we should look instead to the audience to try to understand what defines radio in 2005. Is it the perceived communality of the listening experience? Or the concept of 'liveness'? Is genuine interactivity between listener and broadcaster the new hallmark of radio?
One thing is clear; our current definitions of radio are inadequate. Perhaps the time has come to redefine radio for the digital age? Or then again, perhaps not. Perhaps attempting to define radio is to miss the point, failing to acknowledge its ultimately ethereal nature. Maybe we should put the semantics to one side and be content to enjoy the manifold pleasures of a medium assured of a bright digital future.