What is the difference between an oscilloscope and a spectrum analyzer/signal analyzer?

We usually use time as a frame of reference, focusing on when certain specific events will occur. This includes electrical events. Oscilloscopes allow you to view the instantaneous value of a particular electrical event (or the voltage value of some other event converted by an appropriate transducer) over time. In other words, we can use an oscilloscope to view the waveform of a signal in the time domain. Fourier theory tells us that any electrical phenomenon in the time domain consists of one or more sinusoids with the appropriate frequency, amplitude, and phase. This means that we can convert a time-domain signal to its frequency-domain equivalent. Spectrum analyzers and signal analyzers can measure the energy at each specific frequency in the frequency domain.