In this case I usually suggest students look at an online dictionary. However this usually results in their being confused about words in the definition. Alternatively students look in a thesaurus and then end up with words in their essays with completely different meanings than the ones originally intended.
Wordsift is a visual dictionary that gives pictures and a 'mind-map' of related words to get around this. It was particularly helpful for Evolution, producing the following screen:
The pictures on the left, taken from google, gave an image that students immediately identified with ("Ah, monkey into men!") and the box on the right gives a series of words. Hold your mouse over them and a plain english definition appears. Click on the other words and a whole new word constellation begins. The map clearly shows how closely related the synonyms are and gives students a chance to find out their meaning by holding the mouse over them.
Great for showing the whole class when stuck with a concept.