A commitment to a future act. Also a placeholder for a result not yet arrived — both are trust extended forward in time.
Latin promissum (a thing promised), from promittere (to send forth, to assure beforehand): pro- (forward) + mittere (to send); a promise is trust sent ahead of the act that will either meet it or fail to.
A commitment to a future act, or a placeholder standing in for a result not yet arrived. Both senses send something forward in time before it is earned, which is exactly what the root describes.