Infeudation is an archaic word that refers to the granting of land to a vassal in return for the vassal's loyalty and service. It is not a word that is commonly used in modern English, and as such, there are relatively few synonyms for it.
One possible synonym is "feudalism," which refers to the social and economic system of medieval Europe in which lords granted land to vassals in exchange for their loyalty and service. Other synonyms might include "feudal tenure," "feudal grant," or "land grant."
While these terms are not exact synonyms for "infeudation," they share a similar meaning and context. They all relate to the complex system of land ownership and social hierarchy that characterized medieval Europe and other feudal societies around the world.