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+# ![Unist](https://cdn.rawgit.com/wooorm/unist/master/logo.svg)
+
+**Unist** (universal syntax tree) is the combination of two project, and more
+to come, which are the summation of at least [two](https://github.com/wooorm/retext/commit/8fcb1ff4874d1891791280d63125d27ed29b58a8)
+[years](https://github.com/wooorm/mdast/commit/19585b8) of my work and the
+current epitome of that.
+
+It’s basically a system for processing input: parsing it into a syntax tree,
+transformation by plug-ins, and compiling the syntax tree to something else.
+
+This document explains some terminology relating to
+[**retext**](https://github.com/wooorm/retext),
+[**mdast**](https://github.com/wooorm/mdast), and their related projects.
+
+## Unist nodes
+
+**Unist nodes**:
+
+*   must have a `type` property set to a to its namespace semantic
+    `string`;
+
+*   must have either a `value` property set to a `string` or a `children`
+    property set to an array of one or more `Unist` nodes;
+
+*   may have a `data` property set to a `JSON.stringify`able object;
+
+*   may have an `attributes` property set to a an object of `String`ifyable
+    values;
+
+*   may have a `position` property set to a an object containing `start` and
+    `end`, both of which contain an object with `line` and `column` set
+    to an integer referencing their respective (1-based) line and column
+    in the input file. Both may have an `offset` property set to its
+    index from the beginning of the input. If a node represents something not
+    available in the original input, it must not have a `position`.
+
+See [**nlcst**](https://github.com/wooorm/nlcst) for more information
+on **retext** nodes, and [`mdast/doc/nodes.md`](https://github.com/wooorm/mdast/blob/master/doc/nodes.md)
+for information on **mdast** nodes.
+
+## Unist files
+
+**Unist files** are virtual files (such as [**vfile**](https://github.com/wooorm/vfile))
+representing content at a certain location. They are not limited to existing
+files. Neither are they limited to the file-system only.
+
+## Unist utilities
+
+...are function which work with **unist nodes** or **unist files**, agnostic
+of either **mdast** or **retext**.