a diplomatic building where ambassadors live or work
"The new embassy was officially opened to house our ambassador and his staff in the capital city."
an ambassador and his entourage collectively
"The entire embassy traveled to the capital to negotiate the new trade agreement."
The function or duty of an ambassador.
"The primary embassy of our new diplomat was to foster peace between the two nations through continuous dialogue."
In plain English: An embassy is an official building where representatives from another country live and work to help their own government with its citizens abroad.
"The ambassador met with local leaders at their embassy to discuss trade agreements."
The word embassy comes from the Old Italian term for a message or official mission. It entered English through Middle French and Old French before becoming the modern variant we use today.