noun

• (botany)
  "This ſvveete ſmelling Reede is of a darke dun colour, full of ioints and knees eaſie to be broken into ſmall ſplinters, hollovv and full of a certaine vvhite pith, cobvveb vviſe, ſomevvhat gummie in eating, and hanging in the teeth, and of a ſharpe bitter taſte."
• (botany)
• (by extension)
• (by extension)
  "Becauſe many do hold this opinion that this diſeaſe doth conſume the marrovv of the backe: […] [s]ome againe, do tvvine out the pith of the backe vvith a long vvire thruſt vp into the horſſes head, and ſo into his necke and backe, vvith vvhat reaſon I knovv not."

adj

• (mathematics) The ordinal form of the number pi (π; approximately 3.14159…).
  "The pith root of pi is approximately 1.439…"

noun

• (mathematics) One divided by pi, that is, 1/π (approximately 0.31831…).
  "not only that, but your "radian" axis can be labelled as *being* in units of pis, as opposed to the redundancy of 0pi, pi/2, pi etc.; conversely, your circumferential measure can be rational (or units) and your radius can be transcendental (or piths .-)"