a diplomat of the highest rank; accredited as representative from one country to another
"The newly appointed ambassador arrived in Paris to formally represent her nation's interests before the French government."
A minister of the highest rank sent to a foreign court to represent there his sovereign or country. (Sometimes called ambassador-in-residence)
"The newly appointed ambassador arrived in Paris to formally present his credentials and represent the king at the French court."
In plain English: An ambassador is an official representative sent by their country to another nation to handle diplomatic matters and build relationships.
"The country sent an ambassador to talk with their new government about trade agreements."
Usage: Use this word specifically for high-ranking diplomatic representatives who officially negotiate between nations, rather than informal envoys or corporate liaisons. It is often confused with "envoy," which typically denotes a lower rank or temporary mission without the full sovereign authority of an ambassador.
The word ambassador entered Middle English via Anglo-Norman and Italian, originally meaning a person sent on an official mission or service. It ultimately traces back to Celtic roots related to driving around, replacing the earlier Old English term for one who carries messages.