channel delay spread

Difference in time of arrival between first resolvable multipath component and last component.

If the symbol duration is long compared to delay spread, then there is less inter-symbol-interference.

the inverse is related to coherence bandwidth.

Essentially, we are fixing time and studying delay of components in the channel. The opposite is coherence time.

If the symbol duration is greater than delay spread, this is narrowband fading. Otherwise, its wideband fading.