From Longman Dictionary of Contemporary Englishpithpith /pɪθ/ noun [uncountable] 1 
HBPa white substance just under the outside skin of oranges and similar fruit Peel the oranges with a sharp knife to remove all pith.2 HBPa soft white substance that fills the stems of some plants
HBPa white substance just under the outside skin of oranges and similar fruit Peel the oranges with a sharp knife to remove all pith.2 HBPa soft white substance that fills the stems of some plantsExamples from the Corpus
pith• Small wonder that doctors are calling for the return of the parasol and pith helmet.• The blanched pith soaked up the sugar syrup during the candying process and turned out to be spectacular.• Peel 3 more oranges, cutting away all the pith and membrane.• Finally we both got fed up and left the pith on half the peel.• We tried taking out the pith from behind the bud, and we tried leaving it in - it made little difference.• Remove the skin and white pith and eat as it is for a simple, refreshing dessert.• Remove zest from orange and lemon; avoid the bitter white pith.• The only real difference would be that the colonial governors sent to deal with the locals back then usually wore pith helmets.Origin pith Old English pitha 