Merriam-Webster Collegiate Dictionary - pith

 I. noun  Etymology: Middle English, from Old English ~a; akin to Middle Dutch & Middle Low German pit ~, pit  Date: before 12th century  1.  a. a usually continuous central strand of spongy tissue in the stems of most vascular plants that probably functions chiefly in storage  b. any of various loose spongy plant tissues that resemble true ~  c. the soft or spongy interior of a part of the body  2.  a. the essential part ; core  b. substantial quality (as of meaning)  3. importance  II. transitive verb  Date: 1805  1.  a. to kill (as cattle) by piercing or severing the spinal cord  b. to destroy the spinal cord or central nervous system of (as a frog) usually by passing a wire or needle up and down the vertebral canal  2. to remove the ~ from (a plant stem)