Noun

1. An instrument for measuring atmospheric pressure
2. Anything used as a gauge or indicator.
   • 1916. Michigan Law Review, Vol. 14, No. 8 (Jun., 1916), pp. 661-665 http://links.jstor.org/sici?sici=0026-2234%28191606%2914%3A8%3C661%3APOAJO%3E2.0.CO%3B2-C&size=LARGE:

     "An election is not necessarily an accurate barometer of public opinion. There are other ways in which it makes itself felt, through the press, the forum, discussion, and through every other type of communication."