Refers specifically to the outer covering of a tree trunk or branch.
The bark of this tree is very thick.
Do not peel off the bark.
The craftsman carefully peeled the bark off the tree.
The trunk of this tree is covered with thick bark.
Cork oaks are known for having thick bark.
樹脂 (resin, the sticky sap of a tree) sounds very similar to 樹皮 (tree bark) but refers to a completely different substance.
A piece or sheet of 樹皮 that has been peeled off is counted with 枚, as it is thin and flat.
Compound of 樹 (tree) and 皮 (skin, bark).