A passing note is a type of communication that is passed between two people without the knowledge of a third party. The term 'passing note' can be abbreviated to OBI (3 letters) or OBIT (4 letters). It is often used in school settings, where students pass notes to each other during class without the teacher's knowledge. Passing notes can also be used in other contexts, such as in the workplace, where employees may pass notes to each other without their boss's knowledge.