1. （柑橘类水果皮和瓤之间的）衬皮
The pith of an orange, lemon, or similar fruit is the white substance between the skin and the inside of the fruit.

1. the choicest or most essential or most vital part of some idea or experience

e.g. the gist of the prosecutor's argument
the heart and soul of the Republican Party
the nub of the story

Synonym: kernelsubstancecorecentercentreessencegistheartheart and soulinwardnessmarrowmeatnubsumnitty-gritty

2. soft spongelike central cylinder of the stems of most flowering plants