Transmitter-to-Receiver latency is a much-cited metric when discussing radio systems.
Andy Kunz, a member of the Spektrum development team, wrote a couple of posts going into great detail about how latency is measured and what it means. Because RCG posts get hard to search, I'm capturing them here for posterity.
I've moderately edited from these posts and a couple in between (quoting people asking questions).
Thanks Andy!