A standard-bearer or flag-bearer is a person (soldier or civilian) who bears an emblem called a standard, i.e. either a type of flag or an inflexible but mobile image, which is used (and often honoured) as a formal, visual symbol of a state, prince, military unit, etc.
This can either be an occasional duty, often seen as an honour (especially on parade), or a permanent charge (also on the battlefield); the second type has even led in certain cases to this task being reflected in official rank titles such as Ensign, Cornet and Fähnrich.
In the context of the Olympic Games, a flagbearer is the athlete who carries the flag of their country during the Opening and Closing Ceremonies.
Look up standard-bearer in Wiktionary, the free dictionary.
Wikimedia Commons has media related to Standard-bearers.