A mill agent is an individual responsible for overseeing the operations of a mill. There are a few different synonyms that could be used for this term depending on the context. One option might be "mill manager," which refers specifically to someone who is in charge of the day-to-day operations of a mill. Another synonym might be "mill supervisor," which could refer to someone who oversees a specific area or department within the mill. Other possibilities might include "milling engineer" (for someone with an engineering background who is responsible for optimizing mill operations), "millwright" (for someone who specializes in the installation, maintenance, and repair of milling equipment), or simply "miller" (for someone who operates the mill machinery).