One and only Definition 1 of 2Pro Tip 1/3Unique Importance Use when something or someone cannot be replaced and holds special significance. His vintage car was the one and only item he treasured most. Pro Tip 2/3Emotional Weight Brings strong emotional value to the person or item being described. The necklace from my grandmother is my one and only keepsake. Pro Tip 3/3Express Uniqueness Highlights the exclusivity of an event or opportunity. This concert is the one and only chance to see them live.