The phrase "difficult words" refers to words that are challenging or complex in nature, often requiring a deeper understanding and knowledge of language. Full definition
This insight could be used to alleviate reading problems with software that offer translations of difficult words or suggest easier texts as soon as readers experience problems. (sciencedaily.com)
Short and simple definitions automatically appear above difficult words, so you can keep reading without having to stop and look up a word. (goodereader.com)
For older students you might use some more difficult words. (educationworld.com)