Vocative case is a case that marks a noun whose referent is being addressed.
The word Brute in the following example is in vocative case:
Pei, Mario A., and Frank Gaynor. 1954.A dictionary of linguistics. New York: Philosophical Library.
Hartmann, R.R.K., and F.C. Stork. 1972.Dictionary of language and linguistics. London: Applied Science.
Crystal, David. 1980.A first dictionary of linguistics and phonetics. Boulder, CO: Westview.