While working with a student, you may come across a term you're not familiar with in a problem. Don't be afraid to ask your student to explain a term! Make it sound like a concept check: "Can you show me where the index is in this problem?" or "Tell me what you've learned about the slope so far."
Also, remember, the internet is a fantastic resource! Click here for a great online math dictionary. It has explanations for both US and UK terminology.
One thing you'll run into working with MathElf students from around the world is that there are some regional differences in terminology used. This can be really confusing the first time you encounter it! Here are some of the most common terms that come up which have regional differences: