As nouns the difference between watchword and trademark
is that
watchword is a prearranged reply to the challenge of a sentry or a guard; a password or signal by which friends can be known from enemies while
trademark is a word, symbol, or phrase used to identify a particular company's product and differentiate it from other companies' products.
As an adjective trademark is
(informal) distinctive, characteristic, signature.
As a verb trademark is
to register something as a trademark.