Add glossary

Author: wooorm

readme.md

@@ -21,6 +21,7 @@ documents. The latest released version is [`1.1.0`](https://github.com/syntax-tr
     *   [Node](#node)
     *   [Parent](#parent)
     *   [Text](#text)
+*   [Glossary](#glossary)
 *   [Unist files](#unist-files)
 *   [Unist utilities](#unist-utilities)
     *   [List of Utilities](#list-of-utilities)
@@ -127,6 +128,63 @@ interface Text <: Node {
 }
 ```
 
+## Glossary
+
+###### Tree
+
+A **tree** is a node and all of its **descendants** (if any).
+
+###### Child
+
+Node X is **child** of node Y, if Y’s `children` include X.
+
+###### Parent
+
+Node X is **parent** of node Y, if Y is a **child** of X.
+
+###### Index
+
+The **index** of a **child** is its number of preceding **siblings**, or `0` if
+it has none.
+
+###### Sibling
+
+Node X is a **sibling** of node Y, if X and Y have the same **parent** (if any).
+
+The **previous sibling** of a **child** is its **sibling** at its **index**
+minus 1.
+
+The **next sibling** of a **child** is its **sibling** at its **index**
+plus 1.
+
+###### Root
+
+The **root** of an object is itself, if without **parent**, or the **root** of
+its **parent**.
+
+The **root** of a **tree** is any node in that **tree** without **parent**.
+
+###### Descendant
+
+Node X is **descendant** of node Y, if X is a **child** of Y, or if X is a
+**child** of node Z that is a **descendant** of Y.
+
+An **inclusive descendant** is a node or one of its **descendants**.
+
+###### Ancestor
+
+Node X is an **ancestor** of node Y, if Y is a **descendant** of X.
+
+An **inclusive ancestor** is a node or one of its **ancestors**.
+
+###### Head
+
+The **head** of a node is its first **child** (if any).
+
+###### Tail
+
+The **tail** of a node is its last **child** (if any).
+
 ## Unist files
 
 **Unist files** are virtual files (such as [**vfile**][vfile])