Assure - definition of assure by The Free Dictionary Web1. to cause to feel sure or certain; convince: to assure a person of one's love. 2. to promise; guarantee: he assured us that he would come. 3. to state positively or with assurance 4. to make (an event) certain; ensure 5. (Law) chiefly Brit to insure against loss, esp of life 6. (Law) property law another word for convey