The main stem or trunk of a plant or tree—the central axis (stem/trunk).
古い木の幹には立派な苔が生えている。
Moss grows richly on the trunk of the old tree.
幹に深い傷があると、木の寿命に関わることがある。
Deep wounds in the trunk can affect a tree's lifespan.
The shaft or main body of an object such as an arrow or tool—the axial part (shaft).
矢の幹（柄）が曲がっていると、まっすぐ飛ばない。
If an arrow's shaft is bent, it won't fly straight.
An old sense meaning the handle of a tool. In modern usage, other words such as 柄 (handle) are more common.
鍋の幹（柄）をしっかり握って持ち上げた。
I gripped the pan's handle firmly and lifted it.