a carved palm-tree figure used to decorate a sacred building
This word never names a living tree. It names a picture of one: a palm shape cut into the wood or stone of a holy building. Every time it appears, it belongs to a description of such a structure, the walls, doors, and pillars of Solomon's temple in 1 Kings and 2 Chronicles, and the gateways and inner walls of the future temple the prophet Ezekiel is shown. The carvers set these palm figures in rows, usually next to winged guardian figures and open flowers, so the surfaces read almost like a garden worked in relief.
The palm was a fitting choice for this kind of ornament. A stylized palm was a common decorative motif in the ancient world, a sign of life and flourishing, and lining a temple with carved palms gave it the feel of a lush, well-watered place. The word points to that worked motif, the palm as art, rather than to a tree growing in the ground.
The ordinary word for the living palm tree is תָּמָר; this term is its architectural relative, kept for the palm rendered in carved relief. Because the two are so close, English Bibles usually translate both simply as "palm tree," which can hide that this one always means the decoration and not the plant.