Any method of transmitting low-bandwidth data in a high bandwidth by adding a fixed high-bandwidth modulation. Properly done, it is immune to jamming, because the receiver removes the high-bandwidth modulation before attempting to detect the data, and any narrowband carriers are spread in the process.
The direct sequence spread spectrum method uses a linear-feedback shift register sequence. The Fourier transform of an LFSR sequence is a discretized version of the sinc function Eric Weisstein's World of Math with a notch in the middle.
