I would say that in life, a "token" satisfies your definition. From http://onlinedictionary.datasegment.com/word/token:
In reply to:

Something given or shown as a symbol or guarantee of authority or right; a sign of authenticity, of power, good faith, etc. Say, by this token, I desire his company. --Shak.

As far as creative writing, if there is a special term for using a token in the plot, I don't know.