Do you have a list of words for which you'd like definitions, but would rather not have to look up each word individually? Then, the free online version of EasyDefine could be just the thing.
Have you ever taught a literature unit where the author used many words which might be unfamiliar to your students? VocabGrabber might be a big help, especially if the text is in public domain, found on such sites as Project Gutenberg.
A teacher can select and copy an entire chapter of text from the book (up to 100 pages) and paste it in VocabGrabber (https://www.visualthesaurus.com/vocabgrabber/). VocabGrabber will create a list of words likely to be challenging to students, and let the teacher pick from those words as to which would be saved in a list. The option exists to include the word in the sentence where it is used in the book.
Those particular features may require teachers to subscribe for about $20 a year -- and the interface isn't especially easy to cut and paste into a Word document. However, there is another free version which produces similar results found at Vocabulary.com (https://www.vocabulary.com/lists/vocabgrabber).
Note: Teachers might also like Vocabulary.com to create spelling and vocab lists for their classes each week -- such as lists which incorporate words actually found in contemporary news items and journal articles. The subscription site also provides online testing of the various word lists. And don't forget such resources as SpellingCity.com and Quizlet.com to help your students practice their words.