This extension that allows you to quickly look up definitions of words and phrases in a clean inline box. The definitions come from Wiktionary, but have been preprocessed to reduce formatting and provide faster lookup. Hold Ctrl and double-click or select any word to look it up - an inline box will open to the dictionary entry of that word, usually including examples, audio pronunciation and synonyms.
You can also lookup manually-entered queries using a shortcut which displays a small lookup form.
When this extension was first created, and until version 3.0, it used Google Dictionary as the data source. However, I was asked by Google to switch to another source, and after some searching ended up with the ancient Webster 1913 dictionary as the only reliable free source for version 3.0. This caused a major decline in the vocabulary size, so I went looking for a solution. Eventually, I took the English Wiktionary dump, preprocessed it into a form suitable for inline lookup, and hosted it myself for version 4.0. That's the source that has been in use since.
The icon for this extension was created by Paul Davey, and was taken from iconarchive.com per the Creative Commons BY-NC-SA license.