Final answer:
The term for combat between two knights on horseback using lances is called a joust. It was an essential part of medieval tournaments and knightly culture.
Step-by-step explanation:
The form of combat between two knights where they rode on horses and tried to knock each other off with a lance is known as a joust. Jousting was a martial game or knightly tournament held in European medieval times. This form of combat was a popular form of entertainment and part of the chivalric order, often associated with the medieval period. Although typically individual in nature, it could also be seen as part of a larger event known as a tournament, which included other types of martial contests.