Pith
Etymology
From Old English "piþa," of Germanic origin, meaning the soft central tissue of a plant stem. In citrus it names the white inner layer; the figurative "pithy" (concise, going straight to the point) comes from the same idea of the essential center.
noun · /PITH/
The soft, spongy white layer beneath a citrus rind — and, figuratively, the essential core of something.
What it names
The bitter part of an orange cooks work to remove, and lends its name to speech that is “pithy,” cut down to its core.
Example
Peel the orange carefully to leave behind the bitter pith.
Related idea
Sits between zest and flesh: the zest is the flavorful colored skin above it, the pith is the bitter cushion, and only past both do you reach the fruit — a small map of the whole rind.