This page is a list and recommendation of several online English dictionary websites and browser tools. They let you double click on a word in any web page to see its definitions.
If you are using Google Chrome web browser, you can install a extension that lets you double-click on a word in any webpage and its definition will be shown in a pop-up window.
One of Firefox extension i use is QuickWiki. Shift-right-click to lookup a word thru Wiktionary (via pop-up). Control-right-click to lookup Wikipedia.
Download at https://addons.mozilla.org/en-US/firefox/addon/quickwiki/
Firefox has other dictionary add-ons. Search here: search addons.mozilla.org for dictionary.
Here are some popular dictionary sites and their URL search syntax, using sample word “curlicue”.
When choosing a online dictionary, you want one that's fast loading, not many ads, not cluttered, has human voice recording (as opposed to machine pronounced) for many imported words (Bach, fete, sui generis, …), but most of all, contains the definition from a dictionary you like.
Here's a quick summary: i highly recommend American Heritage Dictionary. Its vocabulary size is larger than all the others online. Its definition is professional and rich. It is not dumbed-down “explanatory” style as in learner's dictionaries, nor is it terse and incomprehensible as in the abridged “Merriem-Webster collegiate”, nor is it “web samples” as in Word Net.
The worst is the Merriem-Webster COLLEGIATE dictionary. It is a abridged dictionary and very popular due to i suppose aggressive marketing. Also, extremely bad is the dictionary bundled with every Mac OS X: New Oxford American Dictionary. It is a “learner's dictionary” and a bad one.
“Learner's dictionary” is a class of dictionaries that take a explanatory approach. Instead of giving a definition, it simply explains what the word means. This is great if you are a casual reader and simply want to know a word's meaning quickly, but it has several serious problems.
For in-depth review of dictionaries, see:
If you are a emacs user, see: Emacs: Lookup Google, Dictionary, Documentation.