Formal:
* Comfort: This implies a sense of easing worry or distress.
* Soothe: This emphasizes the calming effect of reassurance.
* Assure: This simply means to make certain or confident.
* Guarantee: This suggests a strong and unwavering promise.
* Affirm: This emphasizes confirming or validating something.
* Bolster: This implies strengthening or supporting someone's confidence.
Informal:
* Calm: This is a simple and direct synonym.
* Set at ease: This emphasizes the removal of anxiety.
* Ease the mind: This focuses on the mental relief provided.
* Give someone confidence: This highlights the positive impact of reassurance.
The best synonym for you will depend on the specific situation and the intended tone.