1. First and foremost, the Merriam Webster provides a definition, transcription and examples of using. Moreover, it gives a list of other grammar terms, the origin of the word and a rhymes.
2. As an alternative, you may also use Urban dictionary to identify the meaning of the word. Besides you can study the word in detail - it's origin, different forms and so on, and find an examples of different slang and offensive words.
3. Another interesting online dictionary is Etymology dictionary which can help you to observe the changes of the meaning of the word through the time. By the way, it’s a viable option that can help you to expand your vocabulary in a short period of time.
4. As for online tools, I would advise you to use Graph words. Because of the representation of the connection between the words you can learn English with a remarkable ease. It will give you an opportunity to explore a wide range of vocabulary.
5. The last but not the least, is Synonym Finder. It will really help you in writing essays. It contains lots of synonyms of every word.
I’ll be happy if you adopt these methods.