A declaration that one will or will not do a certain thing

• word of honor
• word
• warrant
• vow
• solemn word
• plight
• pledge
• guaranty
• guarantee
• engagement
• covenant
• assurance

To assume an obligation

• undertake
• pledge
• engage
• contract

To guarantee by a solemn promise

• vow
• swear
• plight
• pledge
• covenant

A pledge

• foretaste
• warranty
• parole
• word of honor
• attestation
• gentleman-s-agreement
• giving one's word
• marriage-contract
• plighted faith
• espousal
• affiance
• betrothal
• commitment
• stipulation
• obligation
• insurance
• guarantee
• profession
• vow
• troth
• word
• plight
• swearing
• affirmation
• asseveration
• warrant
• avowal
• consent
• covenant
• engagement
• oath
• pact
• agreement
• assurance

Hope

• good appearance
• good-omen
• sign
• outlook

To give one's word

• cross one's heart and hope to die
• pledge one's honor
• answer-for
• lead one to expect
• subscribe
• underwrite
• give security
• secure
• make oneself answerable
• obligate oneself
• commit oneself
• bind oneself
• plight one's troth
• plight one's faith
• live up to
• keep a promise
• cross-one-s-heart
• insure
• give warranty
• give assurance
• warrant
• guarantee
• assure
• betroth
• affiance
• bargain
• contract
• covenant
• pledge
• undertake
• profess
• affirm
• asseverate
• consent
• espouse
• swear
• vow
• agree
• declare
• engage
• plight

To appear promising

• assure
• insure
• ensure