On an exam, you can often bring one A4 sized piece of paper that you can write anything on. What's that piece of paper called in this context?
Such a reference is known as a cheatsheet or your crib notes.
Note that the paper may be called a cheatsheet even if its use is sanctioned and you're not cheating by using it. Similarly, a crib note doesn't necessarily need to be cribbed.
A cheat sheet or crib sheet is a concise set of notes used for quick reference. "Cheat sheet" may also be rendered "cheatsheet".
Cheat sheets are so named because they may be used by students without the instructor's knowledge to cheat on a test. However, at higher levels of education where rote memorization is not as important as in basic education, students may be permitted to consult their own notes during the exam (which is not considered cheating). The act of preparing a so-called cheat sheet is also an educational exercise, thus students are sometimes only allowed to use cheat sheets they have written themselves. In such usage, a cheat sheet is a physical piece of paper, often filled with equations and/or facts in compressed writing. Modern students often print cheat sheets in extremely small font, fitting an entire page of notes in the palm of their hands during the exam.