A dictionary based on word occurrence frequencies

Solution 1:

wordfrequency.info has, it says, "simple word lists of the top 5,000-60,000 words" and "word lists + genre frequency". Some lists are available free at the site, others have fees.

Solution 2:

I don't know much about on-line tools, but most dictionaries highlight whether the term we are looking up is very frequent or not, whether it is academic or everyday language, etc. For example, Longman Exams Dictionary marks the 3000 most important words to know using a blue lozenge and adding S1, S2 or S3 to show that the word is one of the 1000/2000/3000 most frequent words in spoken English, and W1, W2 and W3 for written English. Instead, the label AC indicates an Academic word, which might be useful to know when writing an assignment.

Other dictionaries have a different code but they supply more or less the same kind of information. These dictionaries are available as online resources as well (sometimes you need to sign up for them).

UPDATE This is a link to Oxford Advanced Learner's Dictionary Online. Under the heading "Wordlists" you can find a list for the 3000 most used words, then you have something similar for academic vocabulary and also the possibility to see illustrations for the words of your choice.

On the other hand, if you just want to have word lists, you may try this link. The downside of it is that terms are just listed, and you must look for definitions elsewhere, but the bright side is that they have a variety of lists, from very basic to more articulate ones, both for British and American English. There is also a list of frequent adjectives and nouns.