When you adjust your material, you can select to add individual terms. First search for the word, if you can’t find it, you can choose to add to your materials and add definitions on the buttom.
1. The first one was mentioned by the poster above. If you tap the settings icon for your schedule, you'll see the Adjust Materials link. In there, you can either add from lessons in the lesson center, or switch over to "add individual terms" and search for things there. Anything that isn't already in the dictionary will give you a prompt for making a custom (user-made) term.
You can do the same thing in the lesson center (under Resources) if you want to make a standalone lesson (both for yourself, and for others). Just tap the "Add lesson" button on that page!