Relative to a reference signal, channel-to-channel skew is the variation in time of the position of the rising edge between the two signals. Here is National Instrument's definition.
It is also important to understand the concept below.
For Saleae Logic products, the worst case channel to channel skew is 1 sample.
If recording the same signal with two different channels, the reason they may be off by 1 sample is due to small differences in the propagation delay, both inside and outside of the logic analyzer, and differences in the threshold voltage between the two channels.