Tuber or bulb is a term used to describe a type of plant that grows from an underground stem, such as potatoes, onions, garlic, and tulips. The crossword solution for the term 'tuber or bulb' is 'SEED, ' which refers to the small, hard, dry structure that contains the reproductive parts of a plant. Seeds are the primary way in which plants reproduce and spread, and they are essential for the growth of new plants.