pledge

Definition:

The term "pledge" refers to formally and solemnly promising to do something or give something.

In the Old Testament the officials of Israel pledged to be loyal to King David.
The object given as a pledge would be returned to its owner when the promise was fulfilled.
"To pledge" could be translate as, "to formally commit to" or "to strongly promise."
The term "pledge" can also refer to an object given as a guarantee or promise that a debt will be paid.
Ways to translate "a pledge" could include "a solemn promise" or "a formal commitment" or "a guarantee" or "a formal assurance," depending on the context.

(See also: promise · oath · vow)