|Index cards for memorization|
This old-fashioned tool used to be handwritten on the front and back of 3 by 5 index cards, but no longer. Now there are excellent e-versions which can include not only definitions but also pictures, sounds, and fill-in-the-blank questions. These modern flash cards are available as apps for iOS and Android devices. You can study anywhere!
This tool is best for memorization and recall. Learning vocabulary in a new language, or multiplication and division tables, or chemical formula names, or famous painting or musical selection identification, or physics or mathematics formulas, or anatomical names, or… pretty much anything you need to commit to memory.
Several companies offer pre-made flashcards on a wide variety of topics, or you can create your own sets and then trade them. Some offer free services, others charge a fee. Many have created games to make studying more fun. Most keep stats so you can track your progress. The best flashcard sets optimize your memorization by repeating incorrect guesses later in your study session, and waiting to repeat correct responses for some set amount of study later on.
Here are some examples of companies offering flash card sets:
Give flash cards a try, no matter what you are studying.