Lingua.ly is a great extension built for developing vocabulary in a new foreign language but also works great to develop vocabulary for struggling students.
The concept is simple and works great. An account is created with an initial set up process. The student chooses their native language and language to learn (for this case both are set to English) and once set up is complete the student is ready to read any web content.
When a student lands on an unfamiliar word, they may double click the word. A simple definition comes up and the word is pronounced for the reader. The word then remains highlighted on the webpage.
For further review and practice, students can log into their Lingua.ly account and either read new content based on the words previously selected or practice flash card type games with these collected words.
Here is a video highlighting the basic functions: