Audio is a signal, any kind of sonic representation being a function of some sort. Signals in the time domain are conceptualized as streams of information against the constant flow of time. However, audio signals are not necessarily time-domain specific. In the time domain, digital signals are functions of discrete (that is, not continuous) amplitude values per unit time (measured in samples). A constantly updated graph of these values (typically in the vertical axis) against time is just a visualization of this signal's waveform, which is therefore a time-domain specific representation.
To understand what exactly a waveform represents, consider how audio signals are translated to physical sound and vice versa. To represent physical sonic waves in the digital domain, we first need to convert them into fluctuations of electrical potential (that is, voltage) using some kind of a transducer (for instance, a microphone) and then convert this alternating voltage into...