Real Dictionary

pledge
When someone makes a pledge, they make a serious promise that they will do something.
verb: If you pledge a sum of money to an organization or activity, you promise to pay that amount of money to it at a particular time or over a particular period.
verb: When someone pledges to do something, they promise in a serious way to do it. When they pledge something, they promise to give it.

pledge in American English
the condition of being given or held as security for a contract, payment, etc.
noun: a solemn promise or agreement to do or refrain from doing something

pledge in British English
noun: a formal or solemn promise or agreement, esp to do or refrain from doing something