General Synonyms:
* Promise
* Vow
* Guarantee
* Assurance
* Commitment
* Undertaking
* Agreement
* Compact
* Covenant
* Word
* Oath
Synonyms for "pledge" in a financial context:
* Deposit
* Security
* Collateral
* Guarantee
* Bond
Synonyms for "pledge" in a ceremonial context:
* Vow
* Oath
* Sacrament
* Dedication
Synonyms for "pledge" in a romantic context:
* Promise
* Commitment
* Vow
* Devotion
More specific synonyms:
* Solemn pledge: vow, oath
* Formal pledge: agreement, covenant
* Informal pledge: promise, word
* Conditional pledge: guarantee, assurance
* Unconditional pledge: commitment, undertaking
The best synonym for "pledge" will depend on the specific context in which it is being used.