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Webster 1913 Edition

Noun.

[F.

postillon

, It.

postiglione

, fr.

posta

post. See

a postman.]

One who rides and guides the first pair of horses of a coach or post chaise; also, one who rides one of the horses when one pair only is used.

[Written also

postillion

The man on one of the front horses is a postilion

postilion ‎(plural postilions)

A rider mounted on the near (left) leading horse who guides the team pulling a carriage.
(obsolete) A post-boy, a messenger boy, a swift letter carrier.

rider mounted on leading horse