pith

The soft, spongy substance inside plant parts; specifically, the parenchyma in the centre of the roots and stems of many plants and trees.

The albedo (“whitish inner portion of the rind”) of a citrus fruit.

The soft tissue inside a human or animal body or one of their organs; specifically, the spongy interior substance of a horn or the shaft of a feather.

Chiefly of animals: the soft tissue inside a spinal cord; the spinal marrow; also, the spinal cord itself.

Synonym of diploe (“the thin layer of soft, spongy, or cancellate tissue between the bone plates which constitute the skull”).

The soft tissue of the brain.

The soft inner portion of a loaf of bread.

The central or innermost part of something; the core, the heart.

The essential or vital part of something; the essence.

Physical power or strength; force, might.