Which of the following describes a signal in the time domain?

A signal represented in the time domain plots how the amplitude of the signal changes continuously over time. This is exactly what you see when you look at a standard oscilloscope display, where the horizontal X-axis represents time and the vertical Y-axis represents amplitude (usually voltage). By contrast, a signal represented in the frequency domain, such as on a spectrum analyzer, plots amplitude on the Y-axis against frequency on the X-axis. It is important to remember that standard time-domain analysis focuses purely on instantaneous amplitude values at specific moments, making options like 'frequency at different times' or 'power at intervals of phase' incorrect for describing a basic time-domain signal.