Have a look at the range of dictionaries on page
http://library.trinity.wa.edu.au/subjects/english/dict.htm

Dictionary.com has an easy to use dictionary

Google has its own dictionary and can translate from another language. Go to Google and place this in the search box .... define: ______-word [ See here]

Easy Dictionary with pictures

Another free dictionary

http://www.factmonster.com/dictionary.html
  (took ages to open)

www.onelook.com
(more for older students)

http://www.wordcentral.com/
  (the one I chose to use)

http://www.wordsmyth.net
 (my second choice)

http://merriam-webster.com/  
(dictionary & thesaurus - maybe a bit above Yr3)

http://www.macquariedictionary.com.au/
 (search facility gave no result for 'bombastic')

Try Word's thesaurus: a chance to introduce keyboard shortcuts like Shift+F7 or right click the word (it has to be in a sentence) and select synonym from the shortcut menu. (option if online not available)
 
http://www.rcls.org/ksearch.htm
(clean interface to search a number of sources)