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A leaf of a tree is any node which has degree of exactly 1. Put another way, a leaf node of a rooted tree is any node which has no child nodes.
$$\xymatrix{ & \bullet \ar@{-}[dl] \ar@{-}[dr] & & & \\ {\color{red}\bullet} & & \bullet \ar@{-}[dr]\ar@{-}[dl] & & \\ & \bullet \ar@{-}[dl] & & {\color{red}\bullet} & \\ {\color{red}\bullet} & & & & }$$
Figure: A tree with leaf nodes highlighted in red.
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