Here are two quick ways how to help your students get the meaning of an unknown word provided they have a personal PC.
Lingro before and it is worth mentioning it again. You have 2 options for using Lingro.
Either you just go to the Lingro website and paste the address of the webpage in the address box, then Lingro will open this page where each word will be clickable and word explanations will be displayed. Or you can install Lingro as an add-in to Firefox browser and have it at your disposal on any webpage you visit.
It is extremely useful to older students who can understand the meaning of a word by reading its definition, not only by translating it into their mother tongue. A superb way of one's vocabulary development.
Another possibility is to add an extension Dictionary Bubble to your Chrome browser and be able to double-click ANY word on the page you have opened. The application will show you the definition of the word and also its synonyms. There is a transcript and an audio pronunciation available for each word.
Students won't have to look for any other dictionary (and they tend to be a bit lazy, don't they?) and they won't have to make excuses for not understanding the difficult words.