1 to make a formal, usually public, promise that you will do something
promise
pledge something to something/somebody
Moore pledged $100,000 to the orchestra at the fund-raising dinner.
Moore pledged $100,000 to the orchestra at the fund-raising dinner.2 to make someone formally promise something :
make somebody promise
3 to leave something with someone as a pledge1 (3)
leave something
4SEC to promise to become a member of a fraternity or sorority at an American university
