n. 1. Bot. The soft spongy parenchymatous center of the stems of most flowering plants. 2. Zool. The soft inner substance of a feather or hair. 3. The essential or central part; the heart or essence. 4. Strenth; vigor; mettle. 5. Significance; importance.
Today, fifty-two years later, its vigor is unimpaired, and for sheer pith I think it probably sets a record that is not likely to be broken.
E.B. White, Introduction to The Elements of Style
Andrea's Lexicon 