pith (http://definr.com/pith)

     n 1: soft spongelike central cylinder of the stems of most
          flowering plants
     2: the choicest or most essential or most vital part of some
        idea or experience: "the gist of the prosecutor's
        argument"; "the heart and soul of the Republican Party";
        "the nub of the story" [syn: kernel, substance, core,
         center, essence, gist, heart, heart and soul, inwardness,
         marrow, meat, nub, sum, nitty-gritty]