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.
![$\displaystyle \begin{xy} *!C\xybox{ \xymatrix{ & \bullet \ar@{-}[dl] \ar@{-}[dr... ...-}[dl] & & {\color{red}\bullet} & \ {\color{red}\bullet} & & & & } } \end{xy}$](http://images.planetmath.org:8080/cache/objects/2741/l2h/img1.png "$\displaystyle \begin{xy} *!C\xybox{ \xymatrix{ & \bullet \ar@{-}[dl] \ar@{-}[dr... ...-}[dl] & & {\color{red}\bullet} & \ {\color{red}\bullet} & & & & } } \end{xy}$")