Is latency equivalent to RTT?

Has it become common usage to define latency in an IP network as the round trip time in that network? I've always considered latency to be a one-way measure of delay and RTT to be the sum of the latencies in both directions. When I tried to find something to back up this view, I discovered that a number of companies define latency as equivalent to RTT in their SLAs. Assuming that one has measuring devices in every PoP, do you think it is harder to measure a full matrix of one way latency compared to measuring a full matrix of RTT? Does it even make sense to measure a full matrix of RTT when the measurement of A to B to A should be equivalent to the measurement of B to A to B? --Michael Dillon