A Tiddler is the fundamental piece of content in a TiddlyWiki. You are reading a Tiddler right now. A Tiddler can contain human readable content, data, or javascript code that makes up a plugin, macro or command.

In basic TiddlyWiki Tiddlers are limited to living in the one HTML file that makes up the TiddlyWiki. With TiddlyWeb Tiddlers have broader scope:

• They have their own unique URL on the web, which gives them option of travelling around the internet to be used by multiple people in different TiddlyWikis.
  
• They can contain any content at all. If you can represent it with a MIME type and a byte stream, you can put it in a tiddler in TiddlyWeb.
  
• They can be (re-)used in many contexts.