1 : a cover for holding together sheets of paper
a loose-leaf binder
2 : a material that is used to hold things together
The egg in the recipe acts as a binder.
3 : a person or machine that puts books together
4 US : a temporary insurance contract that provides coverage until a policy is issued
5 US : a payment given to make an agreement official and legal
She gave the lawyer 1,000 dollars as a binder when she signed the agreement to buy the house.