The Roman military standard carrying an eagle as its symbol was called the aquila.
The aquila was the principal standard of a Roman legion. It usually consisted of a metal eagle mounted on a pole, and the standard-bearer who carried it was known as the aquilifer. Losing a legion’s eagle was considered a grave disgrace because the object represented the unit’s identity, honor, and relationship with Rome.
Roman soldiers guarded the aquila in camp and followed it during campaigns. Ancient writers describe exceptional efforts to recover captured standards, including Augustus’s diplomatic recovery of standards lost by Roman forces to the Parthians in 53 BCE.
The aquila should not be confused with every Roman military flag. A vexillum was a square banner, while a signum was a different unit standard. The eagle became closely associated with Roman imperial power, although specific designs and materials varied over time.