在计算机科学中，二叉树（英语：Binary tree）是每个节点最多有两个子树的树结构。通常子树被称作「左子树」（left subtree）和「右子树」（right subtree）。二叉树常被用于实现二叉查找树和二叉堆。

二叉树的每个节点至多只有二棵子树(不存在度大于2的节点)，二叉树的子树有左右之分，次序不能颠倒。二叉树的第i层至多有个结点；深度为k的二叉树至多共有个结点；对任何一棵二叉树T，如果其终端结点数为，度为2的节点数为，则。

一棵深度为k，且有个节点称之为满二叉树；深度为k，有n个节点的二叉树，当且仅当其每一个节点都与深度为k的满二叉树中，序号为1至n的节点对应时，称之为完全二叉树。

In computer science, a binary tree is a tree data structure in which each node has at most two children, which are referred to as the left child and the right child. A recursive definition using just set theory notions is that a (non-empty) binary tree is a triple (L, S, R), where L and R are binary trees or the empty set and S is a singleton set. Some authors allow the binary tree to be the empty set as well.