Final answer:
Lesser lords who served powerful lords in medieval Europe were called vassals; they were sworn to support their lord in exchange for land and protection.
Step-by-step explanation:
The lesser lords who provided service to powerful lords during the medieval period were known as vassals. These vassals entered into a feudal contract providing military service, counsel, and other obligations in exchange for a grant of land, or a fief. They played a critical role in the feudal system, which was a hierarchical structure of land tenure and obligation that dominated European society.