The Collaborative International Dictionary of English v.0.48:
Most-favored-nation clause \Most-favored-nation clause\
A clause, often inserted in treaties, by which each of the
contracting nations binds itself to grant to the other in
certain stipulated matters the same terms as are then, or may
be thereafter, granted to the nation which receives from it
the most favorable terms in respect of those matters. It is
used most frequently in treaties regarding the terms of trade
between countries, as regarding tariffs and non-tariff
barriers to trade.
[Webster 1913 Suppl. +PJC]
There was a "most-favored-nation" clause with
provisions for the good treatment of strangers entering
the Republic. --James Bryce.
[Webster 1913 Suppl.]
Steam navigation was secured by the Japanese as far as
Chungking, and under the most-favored-nation clause the
right accrued to us. --A. R.
[Webster 1913 Suppl.] Mostic