diff --git a/src/Configurables/DefinitionBook.php b/src/Configurables/DefinitionBook.php
new file mode 100644
index 0000000..0e73db5
--- /dev/null
+++ b/src/Configurables/DefinitionBook.php
@@ -0,0 +1,54 @@
+<?php
+
+namespace Erusev\Parsedown\Configurables;
+
+use Erusev\Parsedown\Configurable;
+
+/**
+ * @psalm-type _Data=array{url: string, title: string|null}
+ */
+final class DefinitionBook implements Configurable
+{
+    /** @var array<string, _Data> */
+    private $book;
+
+    /**
+     * @param array<string, _Data> $book
+     */
+    public function __construct(array $book = [])
+    {
+        $this->book = $book;
+    }
+
+    /** @return self */
+    public static function default()
+    {
+        return new self;
+    }
+
+    /**
+     * @param string $id
+     * @param _Data $data
+     * @return self
+     */
+    public function setting($id, array $data)
+    {
+        $book = $this->book;
+        $book[$id] = $data;
+
+        return new self($book);
+    }
+
+    /**
+     * @param string $id
+     * @return _Data|null
+     */
+    public function lookup($id)
+    {
+        if (isset($this->book[$id])) {
+            return $this->book[$id];
+        }
+
+        return null;
+    }
+}
diff --git a/src/Parsedown.php b/src/Parsedown.php
index 6373805..99674e7 100644
--- a/src/Parsedown.php
+++ b/src/Parsedown.php
@@ -42,9 +42,6 @@ class Parsedown
 
     protected function textElements($text)
     {
-        # make sure no definitions are set
-        $this->DefinitionData = [];
-
         # iterate through lines to identify blocks
         return $this->linesElements(Lines::fromTextLines($text, 0));
     }
@@ -1739,12 +1736,6 @@ class Parsedown
 
     private static $instances = [];
 
-    #
-    # Fields
-    #
-
-    protected $DefinitionData;
-
     #
     # Read-Only