I have trouble with bigger words, and I know other people sometimes have to look up words that they don't know. I had the word "expenditures" in a word problem for a table, I didn't even know that was a word or what it means in the first place. 

I think that a Vocab box on Khan Academy would be a good idea knowing that people have trouble with words and what they mean if they haven't seen the word or know what it means off the top of their head.