To make one feel confident or assured about something; to remove one's doubt or misgivings about something. I tried to reassure Sarah about her relationship with Tom, but I could tell she wasn't convinced. You'll need to reassure them about the security of their investment after such a disappointing fiscal quarter.

To make one feel confident or assured about something; to make a promise or guarantee about something to one. They went to great lengths to reassure me of the security of my job after the completion of the merger. He reassured me of his devotion to our marriage.