When writing for young children, I often need simple definitions to include in the text or in the glossary. Some publishers have their own preference for dictionary usage. In my most recent biographies for Rourke Publishing, my editor wanted all the definitions and pronunciation taken from the Scholastic Children's Dictionary. I have several dictionaries on my shelf, but I didn't have that one, so I ordered it from Amazon and added it to my collection. But I often use online dictionaries while I'm writing.
These are my favorites:
I usually start with Wordsmyth . It gives me the option to choose from beginner, children's, or advanced definitions. It also has a box that shows the words that come before and after a chosen word, like looking at a page in a print dictionary and a box for multi-word combinations.
If I'm not completely satisfied with my definition, I'll check Word Central ,which is powered by Merriam-Webster and Yahoo Kids Dictionary with references from the American Heritage Dictionary. Sometimes it takes a combination of several definitions for me to settle on the best one for a particular project.
I recently discovered Fact Monster. While the information here is written at a higher level, I like the way this website offers multiple definitions of the word as different parts of speech.
RhymeZone requires an extra step if you're looking primarily for a definition. A drop down menu beside the word entry lets you search for rhymes, near rhymes, synonyms, antonyms, homophones, definitions, quotations, and a few other choices. It even gives you the option to search for your word in the definition of another word, like a reverse dictionary.