pith
/pɪθ; pɪθ/ n
1 [U]
(a) soft spongy substance that fills the stems of certain plants, eg reeds （木）髓（某些植物茎中的海绵样组织, 如芦苇的茎髓）.
(b) similar substance inside the skin of oranges, etc （柑橘等果皮肉的）类似髓的组织.
2 [sing] (fig 比喻) the ~ of sth most important or essential part of sth; essence 最重要部分; 精髓
That was the pith of his argument. 那就是他论据的重点.