The Collaborative International Dictionary of English v.0.48:

Protactic \Pro*tac"tic\ (pr[-o]*t[a^]k"t[i^]k), a. [Gr.
   protaktiko`s placing or placed before, fr. prota`ssein to
   place in front; pro` before + ta`ssein to arrange.]
   Giving a previous narrative or explanation, as of the plot or
   personages of a play; introductory.
   [1913 Webster]