Real Dictionary

pith
uncountable noun: The pith of an orange, lemon, or similar fruit is the white substance between the skin and the inside of the fruit.

pith in American English
the soft, spongy tissue in the center of certain plant stems
to remove the pith from (a plant stem)
noun: the soft, spongy central cylinder of parenchymatous tissue in the stems of dicotyledonous plants
transitive verb: to remove the pith from (plants)

pith in British English
to destroy the brain and spinal cord of (a laboratory animal) by piercing or severing
noun: the soft fibrous tissue lining the inside of the rind in fruits such as the orange and grapefruit