For now it can be changed to "official defin[i]tion of UTC" or "official [definition] of UTC" in the TZDB copy. (I'd go with the first; the second causes less spell checker grief.)
This error was present in the leap-seconds.3629404800 file as provided by NIST. I have copied Judah Levine on this message so that it might be fixed in their next version.
> +# Some systems implement leap seconds by amortizing the leap second
> +# over the last few minutes of the day. The frequency of the local
> +# clock is decreased (or increased) to realize the positive (or
> +# negative) leap second. This method removes the time step described
> +# above. Although the long-term behavior of the time scale is correct
> +# in this case, this method introduces an error during the adjustment
> +# period both in time and in frequency with respect to the official
> +# defintion of UTC.