A more thorough explanation:

Definition: Fidelitatis sacramentum was an oath of loyalty that a vassal had to take to their lord in medieval times.

Example: When a knight was granted land by a lord, they had to swear an oath of fealty to the lord. This oath was called fidelitatis sacramentum.

Explanation: The example illustrates how fidelitatis sacramentum was an important part of the feudal system. The oath of loyalty ensured that the vassal would serve their lord faithfully and protect their land. In return, the lord would provide protection and support to the vassal.