Pith is the spongy, soft tissue inside the stems of many plants and trees. If you've ever peeled an orange, you've seen pith: it's that white, slightly bitter layer between the colorful outer peel and the juicy fruit inside. In plant stems like those of elder trees or cornstalks, the pith runs down the center like a soft core.

The word also means the essential part of something, the heart of the matter. When a teacher asks you to explain the pith of a story, she wants you to identify what it's really about, its central meaning or main point. If you get to the pith of an argument, you've found its most important point. A pithy comment is short but packed with meaning, like when someone sums up a complex idea in one clever sentence.

Scientists studying ancient plants sometimes find preserved pith in fossils, helping them understand how plants grew millions of years ago. The pith in living plants serves as storage for water and nutrients, and in some species, it helps transport materials through the stem.