pledge vs promise

pledge

verb
  • To make a solemn promise (to do something). 

  • To deposit something as a security; to pawn. 

  • To give assurance of friendship by the act of drinking; to drink to one's health. 

noun
  • A solemn promise to do something. 

  • The personal property so pledged, to be kept until the debt is paid. 

  • A bailment of personal property to secure payment of a debt without transfer of title. 

  • A person who has taken a pledge of allegiance to a college fraternity, but is not yet formally approved. 

  • A drinking toast. 

  • An asset or person temporarily handed over to guarantee the fulfilment of something promised, under threat of permanent loss of the thing handed over; surety, security, hostage. 

  • A promise to abstain from drinking alcohol. 

promise

verb
  • To commit to (some action or outcome), or to assure (a person) of such commitment; to make an oath or vow. 

  • To give grounds for expectation, especially of something good. 

noun
  • A transaction between two persons whereby the first person undertakes in the future to render some service or gift to the second person or devotes something valuable now and here to his use. 

  • A placeholder object representing the eventual result of an asynchronous operation. 

  • Reason to expect improvement or success; potential. 

  • an oath or affirmation; a vow 

How often have the words pledge and promise occurred in a corpus of books? (source: Google Ngram Viewer )