As nouns the difference between tether and stay
is that
tether is a rope, cable etc that holds something in place whilst allowing some movement while
stay is (nautical) a strong rope supporting a mast, and leading from the head of one mast down to some other, or other part of the vessel or
stay can be a prop; a support.
As verbs the difference between tether and stay
is that
tether is to restrict something with a tether while
stay is (nautical) to incline forward, aft, or to one side by means of stays or
stay can be to prop; support; sustain; hold up; steady.
As an adjective stay is
steep; ascending.
As an adverb stay is
steeply.