What is delay skew?

To gain an understanding of delay SKEW and why it is so important, it is first necessary to understand the concept of propagation delay. Measured in nanoseconds per meter of cable, propagation delay is an expression of the amount of time a signal is delayed when propagated through a length of wire. Delay SKEW is the result obtained when comparing the propagation delay between any two wiring pairs within the same cable.

Source: Blog.T3innovation.com