Well, along with the proper definitions from other (cited) legal dictionaries on that page, there's also an unsourced, thesaurus-like entry that does seem to suggest what the page is cited for, though the other definitions on the page make clear that that entry is only accurate in terms of loosely associated concepts and not proper synonyms.
Ok.. I know the discussion is going way off-topic and getting pedantic connotations. To illustrate the point in my original comment, I would like to make a last cite from Britannica.
@dragonwriter: For some reason I can't reply your comments, I guess HN is delaying replies after a while precisely to avoid long off-topic threads (although they are fun :)
The fact that one of the words is a subset of another depending on the context doesn't invalidate the fact that the words are still synonyms when they are used in a specific topic.
Moreover, by what you said before:
> "Exchange" is the critical word. A single promise is extraordinarily different...
I would conclude that you are actually agreed on how I used it in my original comment:
> "and contracts might be dissolved if both sides of the contract agreed. This being an open promise is not a contract but a reaffirmation of a goal"
