Frequency spectrum (continuous case)

The graph of $|\hat f(\omega)|$ against $\omega$ , for a non-periodic signal.

The frequency spectrum of $f$ , in the continuous case, is the Fourier transform $\hat f(\omega)$ itself, viewed as a function of $\omega$ describing how much of each frequency the signal contains -- the direct continuous analogue of the discrete spikes a periodic signal's spectrum shows.

Examples

A sharp pulse has a wide frequency spectrum, spreading its energy across many frequencies at once.