In gardening, the term "dormancy" refers to a perennial plant's state of temporary metabolic inactivity or minimal activity. Plants generally go dormant in response to adverse growing conditions, such as when trees or perennial garden plants go dormant during the cold winter months; or when turf grass goes dormant in a lawn during a period of intense heat or drought. It's important to remember that plants don't die at this time, but are simply in suspended animation. While the outer leaves and above-ground foliage may die back, life still lurks in the roots and core of the perennial plant. The term "dormancy" isn't often used to describe annual plants with a life cycle of a single growing season. Their biology does not include the mechanism for going dormant.
During dormancy, plants stop growing and conserve energy until better cultural conditions present themselves. This happens naturally as seasons and weather changes. And it can also be artificially controlled to store plants for shipping or to get them to flower for particular holidays. Tulip and daffodil bulbs, for example, can be artificially chilled to send them into dormancy, then brought out of dormancy at the desired time to force them into bloom when required--such as for Valentine's Day potted plants or Easter lily displays.
For perennial plants in the ground, there is potential danger if a plant breaks dormancy too soon. Many a perennial has been lost for the growing season when an unseasonable warm spell causes the plant to break dormancy and send up green growth, which is then killed when the weather returns to cold. To prevent this, it's recommended that the ground be kept covered with mulch in the spring, to prevent the dramatic thaw-freeze cycles that can cause this problem.