What is the meaning of pith?

noun

• Soft spongelike central cylinder of the stems of most flowering plants.
• The choicest or most essential or most vital part of some idea or experience.
  • usage: "the gist of the prosecutor's argument"; "the heart and soul of the Republican Party"; "the nub of the story"
  • synonyms: kernel, substance, core, center, centre, essence, gist, heart, heart and soul, inwardness, marrow, meat, nub, sum, nitty-gritty

verb

• Remove the pith from (a plant. ).