fruit of the oak tree: a smooth thin-walled nut in a woody cup-shaped base
"The squirrel buried an acorn under the leaves to eat during winter."
The fruit of the oak, being an oval nut growing in a woody cup or cupule.
"The squirrel buried a large acorn beneath the old oak tree before winter arrived."
The word "acorn" comes from Old English æcern, meaning an oak fruit or nut. It ultimately traces back to the Proto-Indo-European root for berry.