Definition: Dormancy is a state of temporary metabolic inactivity or minimal activity. Plants generally go dormant in response to adverse growing conditions. For example: perennial plants going dormant for the winter or grass going dormant in intense heat. They stop growing and conserve energy until better cultural conditions present themselves. This happens naturally as seasons and weather changes. It can also be artificially controlled to store plants for shipping or to get them to flower for particular holidays. Forcing bulbs and flowering branches in the spring is a way of artificially breaking dormancy.
It's helpful to mulch over perennial plants, once the ground has frozen, to prevent them from breaking dormancy too soon.