Spectrum

Spectrum means the frequencies in relation to time, it's often shown in diagrams where time is on the x-axis and ampltude is at the y-axis, the frequency analyzer (plugins and Hardware like RTW) will show you the "loudness" (or to be exact the energy/level) of each frequeny from about 20 Hertz to 20 kHz (thats what we hear).

in Synthesizers a spectrum is often a waveform that represents one snapshot in time, because a spectrum can not be over a time but at a time. to get a spectrum at least one period of a wave (one cycle) is used.